You are given a string S consisting of lowercase English letters.
Find the lexicographically (alphabetically) smallest lowercase English letter that does not occur in S.
If every lowercase English letter occurs in S, print None instead.

-----Constraints-----
 - 1 \leq |S| \leq 10^5 (|S| is the length of string S.)
 - S consists of lowercase English letters.

-----Input-----
Input is given from Standard Input in the following format:
S

-----Output-----
Print the lexicographically smallest lowercase English letter that does not occur in S.
If every lowercase English letter occurs in S, print None instead.

-----Sample Input-----
atcoderregularcontest

-----Sample Output-----
b

The string atcoderregularcontest contains a, but does not contain b.
[{"code": "S = input()\nalphabet = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']\n\nfor a in alphabet:\n    if a not in S:\n        print(a)\n        return\n        \nprint('None')\n\n\n", "passed": true, "time": 0.15, "memory": 14352.0, "status": "done"}, {"code": "s = input()\nans = 'None'\nfor i in range(97, 123):\n    if chr(i) not in s:\n        ans = chr(i)\n        break\nprint(ans)", "passed": true, "time": 0.15, "memory": 14112.0, "status": "done"}, {"code": "S = input()\nfor c in \"abcdefghijklmnopqrstuvwxyz\":\n    if c not in S:\n        print(c)\n        break\nelse:\n    print(\"None\")\n", "passed": true, "time": 0.4, "memory": 14032.0, "status": "done"}, {"code": "import string\n\nS = set(input())\n\nresult = ''\nfor i in string.ascii_lowercase:\n    if i not in S:\n        print(i)\n        break\nelse:\n    print('None')", "passed": true, "time": 0.17, "memory": 14268.0, "status": "done"}, {"code": "S = input()\nl = sorted(list(set(S)))\n\nans = \"\"\nfor i in range(len(l)):\n  if ord(l[i]) !=  97 + i:\n    ans = chr(97+i)\n    break\n  elif i == 25 and l[i] == \"z\":\n    ans = \"None\"\n\nif ans != \"\":\n print(ans)\nelse:\n  print(chr(ord(l[-1])+1))", "passed": true, "time": 1.04, "memory": 14356.0, "status": "done"}, {"code": "S = str(input())\ns = 'abcdefghijklmnopqrstuvwxyz'\n\nfor i in s:\n  if i not in S:\n    print(i)\n    return\nprint('None')", "passed": true, "time": 1.0, "memory": 14300.0, "status": "done"}, {"code": "import sys\ns = input()\n\nfor i in range(97, 123):\n    st = chr(i)\n    if st not in s:\n        print(st)\n        return\n\n    \nprint(None)", "passed": true, "time": 0.15, "memory": 14292.0, "status": "done"}, {"code": "import string\n\ns = list(map(str, input()))\ns = list(set(s))\ns.sort()\n\nans = \"\"\narr = []\n\nalp = list(map(str, string.ascii_lowercase))\nfor char in s:\n    alp.remove(char)\n\nif len(alp) == 0:\n    print(\"None\")\nelse:\n    print((alp[0]))\n", "passed": true, "time": 0.15, "memory": 14380.0, "status": "done"}, {"code": "import string\ns = input()\nt = set(s) ^ set(string.ascii_lowercase)\n\nfor i in string.ascii_lowercase:\n  if set(i) & t:\n    print(i)\n    break\nelse:\n  print(\"None\")", "passed": true, "time": 0.38, "memory": 14180.0, "status": "done"}, {"code": "S = str(input())\nal = [chr(ord('a') + i) for i in range(26)]\ndata = []\nfor x in S:\n    if not x in data:\n        data.append(x)\nans = list(set(al) ^ set(data))\nans.sort()\nif len(ans) == 0:\n    print('None') \nelse:\n    print(ans[0])", "passed": true, "time": 0.14, "memory": 14076.0, "status": "done"}, {"code": "s = list(input())\ns.sort()\nli =[]\nlis = [97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112,\n 113, 114, 115, 116, 117, 118, 119, 120, 121, 122]\n\nfor i in s:\n    li.append(ord(i))\ncnt = 0\nfor i in lis:\n    if i in li:\n        cnt += 1\n    else:\n        print((chr(i)))\n        return\nprint('None')\n", "passed": true, "time": 0.4, "memory": 14224.0, "status": "done"}, {"code": "A = ['a','s','d','f','g','h','j','k','l','z','x','c','v','b','n','m','q','w','e','r','t','y','u','i','o','p']\nS = list(set(input()))\nfor i in S:\n    A.remove(i)\nA.sort()\ntry:\n    print((A[0]))\nexcept:\n    print(\"None\")\n", "passed": true, "time": 0.15, "memory": 14100.0, "status": "done"}, {"code": "s = input()\n\nalphabets = 'abcdefghijklmnopqrstuvwxyz'\n\nfor i in range(len(alphabets)):\n   if not alphabets[i] in s:\n      print(alphabets[i])\n      return\n\nprint('None')", "passed": true, "time": 0.15, "memory": 14328.0, "status": "done"}, {"code": "import sys\nS = list(input())\ns = list(set(S))\ns.sort()\nalfa = list('abcdefghijklmnopqrstuvwxyz')\nans = 0\nif(len(s)==len(alfa)):\n  print('None')\n  return\nelse:\n  for i in range(len(s)):\n    if(s[i]!=alfa[i]):\n      print(alfa[i])\n      return\n  print(alfa[len(s)])", "passed": true, "time": 0.14, "memory": 14276.0, "status": "done"}, {"code": "import math\nfrom math import gcd,pi,sqrt\nINF = float(\"inf\")\n\nimport sys\nsys.setrecursionlimit(10**6)\nimport itertools\nfrom collections import Counter,deque\ndef i_input(): return int(input())\ndef i_map(): return list(map(int, input().split()))\ndef i_list(): return list(i_map())\ndef i_row(N): return [i_input() for _ in range(N)]\ndef i_row_list(N): return [i_list() for _ in range(N)]\ndef s_input(): return input()\ndef s_map(): return input().split()\ndef s_list(): return list(s_map())\ndef s_row(N): return [s_input for _ in range(N)]\ndef s_row_str(N): return [s_list() for _ in range(N)]\ndef s_row_list(N): return [list(s_input()) for _ in range(N)]\n\nimport string\n\ndef main():\n    a  = string.ascii_lowercase\n    s = s_input()\n    for i in a:\n        if i not in s:\n            print(i)\n            return\n    print(\"None\")\n\n\ndef __starting_point():\n    main()\n\n__starting_point()", "passed": true, "time": 0.15, "memory": 14300.0, "status": "done"}, {"code": "s=input()\nfor i in range(ord(\"a\"),ord(\"z\")+1):\n  if chr(i) not in s:\n    print(chr(i)) \n    return\nprint(\"None\")", "passed": true, "time": 0.14, "memory": 14312.0, "status": "done"}, {"code": "from collections import Counter\n\nS = list(input())\nac = Counter(S)\n\nalp = 'abcdefghijklmnopqrstuvwxyz'\n\nfor a in alp:\n    if not(a in ac):\n        print(a)\n        break\nelse:\n    print('None')\n", "passed": true, "time": 0.14, "memory": 14188.0, "status": "done"}, {"code": "# 35\nimport string\n\nS = str(input())\n\nset1 = set()\nfor s in S:\n    set1.add(s)\n\nset2 = set()\nfor alf in string.ascii_lowercase:\n    set2.add(alf)\n\nlist = sorted(set2 - set1)\nif not list: print('None')\nelse: print(sorted(list)[0])", "passed": true, "time": 0.14, "memory": 14332.0, "status": "done"}, {"code": "S = input()\nalphabets = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']\nans = []\nfor alpha in alphabets:\n    if S.count(alpha) > 0 :\n        ans.append(1)\n    else:\n        ans.append(0)\n\nif sum(ans) == 26:\n    print('None')\nelse:\n    print((alphabets[ans.index(0)]))\n", "passed": true, "time": 0.15, "memory": 14352.0, "status": "done"}, {"code": "s = list(map(str,input()))\nalp = \"abcdefghijklmnopqrstuvwxyz\"\ns = set(s)\nfor i in range(26):\n  if alp[i] not in s:\n    print(alp[i])\n    return\n  elif i == 25:\n    print(\"None\")", "passed": true, "time": 0.22, "memory": 14212.0, "status": "done"}, {"code": "s = input()\n\nalphabets = 'abcdefghijklmnopqrstuvwxyz'\n\nfor x in alphabets:\n    if x in s:\n        continue\n    else:\n        print(x)\n        break\nelse:\n    print('None')", "passed": true, "time": 0.14, "memory": 14116.0, "status": "done"}, {"code": "s = input()\nans = 'None'\nline = [0 for i in range(26)]\nfor i in s:\n  line[ord(i)-97] += 1\nfor j in range(26):\n  if line[j] == 0:\n    ans = chr(j + 97)\n    break\nprint(ans)", "passed": true, "time": 0.15, "memory": 14276.0, "status": "done"}, {"code": "li = list(\"abcdefghijknmlopqrstuvwxyz\")\nS = list(input())\nans = \"None\"\nfor l in li:\n    if not l in S:\n        ans = l\n        break\nprint(ans)", "passed": true, "time": 0.14, "memory": 14164.0, "status": "done"}, {"code": "s = input()\nal = \"abcdefghijklmnopqrstuvwxyz\"\nfor i in al:\n    if i not in s:\n        print(i)\n        break\nelse:\n    print(\"None\")", "passed": true, "time": 0.14, "memory": 14192.0, "status": "done"}, {"code": "s = set(input())\na = \"abcdefghijklmnopqrstuvwxyz\"\nfor si in s:\n    a = a.replace(si,\"\")\nprint(None if a==\"\" else a[0])", "passed": true, "time": 0.14, "memory": 14284.0, "status": "done"}, {"code": "s = list(input())\nimport collections\nimport sys\na = collections.Counter(s)\n\nfor i in range(97,97+26):\n    if chr(i) not in a:\n        print(chr(i))\n        return\nelse:\n    print(\"None\")", "passed": true, "time": 0.15, "memory": 14152.0, "status": "done"}, {"code": "S = input()\n\nimport string\nlst = list(string.ascii_lowercase)\nd = {lst[i]:i for i in range(len(lst))}\ncnt = [0]*len(lst)\n\nfor ch in S:\n  cnt[d[ch]] = 1\n  \nidx = -1\nfor i in range(len(cnt)):\n  if cnt[i] == 0:\n    idx = i\n    break\n\nif idx == -1:\n  print(\"None\")\nelse:\n  print((lst[idx]))\n", "passed": true, "time": 0.3, "memory": 14068.0, "status": "done"}, {"code": "d = {}\nfor s in input():\n  if s in d:\n    d[s] += 1\n  else:\n    d[s] = 1\n\nfor a in 'abcdefghijklmnopqrstuvwxyz':\n  if a not in d:\n    print(a)\n    return\n\nprint('None')", "passed": true, "time": 0.31, "memory": 14048.0, "status": "done"}, {"code": "l = set(\"abcdefghijklmnopqrstuvwxyz\")\ns = set(input())\nans = sorted(list(l^s))\nprint(\"None\" if len(ans)==0 else ans[0])", "passed": true, "time": 0.22, "memory": 14236.0, "status": "done"}, {"code": "s = input()\ns = set(s)\ndata = ['a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z']\nfor i in range(len(data)):\n    if data[i] not in s:\n        print(data[i])\n        break\nelse:\n     print('None')   ", "passed": true, "time": 0.14, "memory": 14068.0, "status": "done"}, {"code": "S=input()\n\nalph=\"abcdefghijklmnopqrstuvwxyz\"\n\nfor i in alph:\n    if i in S:\n        alph=alph.replace(i,\"\")\n\nif len(alph)==0:\n    print(\"None\")\nelse:\n    print(alph[0])", "passed": true, "time": 0.44, "memory": 14100.0, "status": "done"}, {"code": "A = [0 for i in range(26)]\nS = input()\nfor s in S:\n  A[ord(s)-97] = 1\nflag = False\nfor i in range(26):\n  if A[i]==0:\n    flag = True\n    break\nif flag:\n  print(chr(97+i))\nelse:\n  print(\"None\")", "passed": true, "time": 0.31, "memory": 14280.0, "status": "done"}, {"code": "import string\n\ns = input()\nfor x in string.ascii_lowercase:\n\tif x not in s:\n\t\tprint(x)\n\t\tbreak\nelse:\n\tprint('None')", "passed": true, "time": 0.16, "memory": 14088.0, "status": "done"}, {"code": "s=input()\nres=\"None\"\nfor i in range(ord(\"a\"),ord(\"z\")+1):\n    if chr(i) not in s:\n        res=chr(i)\n        break\nprint(res)", "passed": true, "time": 0.32, "memory": 14304.0, "status": "done"}, {"code": "s = input()\ns = list(set(s))\ns = sorted(s)\nalf = [chr(i) for i in range(97,97+26)]\nfor i in range(26):\n    if alf[i] not in s:\n        print((alf[i]))\n        return\n        \nprint('None')\n", "passed": true, "time": 0.32, "memory": 14260.0, "status": "done"}, {"code": "s=input()\nf=1\nfor i in 'abcdefghijklmnopqrstuvwxyz':\n    if i not in s:\n        print(i)\n        f=0\n        return\nif f:print('None')", "passed": true, "time": 0.14, "memory": 14180.0, "status": "done"}, {"code": "S = list(input())\nA = list('abcdefghijklmnopqrstuvwxyz')\nans = 'None'\nfor a in A:\n    if a not in S:\n        ans = a\n        break\nprint(ans)", "passed": true, "time": 0.25, "memory": 14316.0, "status": "done"}, {"code": "c = [chr(i) for i in range(97, 97+26)]\ns = input()\nans = 'None'\nfor i in c:\n    if i not in s:\n        ans = i\n        break\nprint(ans)", "passed": true, "time": 0.14, "memory": 14032.0, "status": "done"}, {"code": "def main():\n    s=input()\n    for x in range(ord('a'), ord('z')+1):\n        c=chr(x)\n        if c in s:\n            continue\n        else:\n            print(c)\n            return\n    print('None')\nmain()", "passed": true, "time": 0.14, "memory": 14196.0, "status": "done"}, {"code": "S = sorted(list(set(input())))\n\nalphabet = list('abcdefghijklmnopqrstuvwxyz')\n\nfor i in range(len(alphabet)):\n    if alphabet[i] not in S:\n        print(alphabet[i])\n        return\n\nprint('None')", "passed": true, "time": 0.14, "memory": 14108.0, "status": "done"}, {"code": "S=input()\na=[0]*26\nfor i in range(len(S)):\n    tmp=ord(S[i])-ord('a')\n    a[tmp]=1\n    \nfor i in range(26):\n    if a[i]==0:\n        tmp=chr(ord('a')+i)\n        print(tmp)\n        break\nelse:\n    print('None')\n", "passed": true, "time": 0.14, "memory": 14096.0, "status": "done"}, {"code": "S = list(input())\n\nS.sort()\n\nif S[0] != 'a':\n    print('a')\nelif S[0] == 'a':\n    for i in range(len(S)-1):\n        if ord(S[i+1]) - ord(S[i]) == 0 or ord(S[i+1]) - ord(S[i]) == 1:\n            pass\n        else:\n            res = ord(S[i]) + 1\n            print((chr(res)))\n            return\n            \n    res = ord(S[-1]) + 1\n\n    if S[0] == 'a' and S[-1] == 'z':\n        print('None')\n    elif S[0] != 'a':\n        print('a')\n    else:\n        print((chr(res)))\n", "passed": true, "time": 0.15, "memory": 14292.0, "status": "done"}, {"code": "# -*- coding: utf-8 -*-\n\nS = set(input())\naz = [chr(i) for i in range(ord('a'),ord('z')+1)]\n\nif len(S) >= 26:\n    print(\"None\")\n    return\n\nfor i in az:\n    if i in S:\n        pass\n    else:\n        print(i)\n        return\n\n\n\n", "passed": true, "time": 0.14, "memory": 14220.0, "status": "done"}, {"code": "s = str(input())\nL =['a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z']\nfor i in s:\n  if i in L:\n    L.remove(i)\nif L == []:\n  print('None')\nelse:\n  print(L[0])", "passed": true, "time": 0.15, "memory": 14084.0, "status": "done"}, {"code": "# -*- coding: utf-8 -*-\n\nS = sorted(set(list(input())))\n\nans = 'None'\nfor keyword in list(\"abcdefghijklmnopqrstuvwxyz\"):\n    hit = False\n    for s in S:\n        if s == keyword:\n            hit = True\n            break \n    \n    if hit == False:\n        ans = keyword\n        break\n\nprint(ans)\n\n", "passed": true, "time": 0.14, "memory": 14104.0, "status": "done"}, {"code": "alphabet = [chr(i) for i in range(97, 123)]\nS = set(list(input()))\nfor s in S:\n  if s in alphabet: alphabet.remove(s)\n    \nprint(alphabet[0]) if alphabet else print(\"None\")", "passed": true, "time": 0.14, "memory": 14180.0, "status": "done"}, {"code": "S = input()\n\nabc = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']\n\n\nfrom collections import defaultdict\n\nd = defaultdict(int)\nfor s in S:\n    d[s] = 1\n\nfor a in abc:\n    if d[a] == 0:\n        print(a)\n        return\nprint('None')", "passed": true, "time": 0.14, "memory": 14092.0, "status": "done"}, {"code": "s = input()\nalphabets = [0] * 26\n\nfor i in s:\n    alphabets[ord(i) - ord(\"a\")] += 1\n\nfor i in range(26):\n    if alphabets[i] == 0:\n        print((chr(i + ord(\"a\"))))\n        break\nelse:\n    print(\"None\")\n", "passed": true, "time": 0.14, "memory": 14340.0, "status": "done"}, {"code": "s = list(input())\nflag = [0] * 26\n\nfor i in range(len(s)):\n  flag[ord(s[i]) - 97] = 1\n\nc = 0\nfor i in range(26):\n  if flag[i] == 0:\n    print(chr(i + 97))\n    break\n  else:\n    c += 1\nif c == 26:\n  print(\"None\")", "passed": true, "time": 0.14, "memory": 14204.0, "status": "done"}, {"code": "S = input()\nN = len(S)\nfor i in range(26):\n    for k in range(N):\n        if S[k] == chr(97+i):\n            break\n    else:\n        print(chr(97+i))\n        return\nprint('None')", "passed": true, "time": 0.14, "memory": 14072.0, "status": "done"}, {"code": "S = input()\nl = sorted(list(set(S)))\n\nans = \"\"\nfor i in range(len(l)):\n  if ord(l[i]) !=  97 + i:\n    ans = chr(97+i)\n    break\n  elif i == 25 and l[i] == \"z\":\n    ans = \"None\"\n\nif ans != \"\":\n print(ans)\nelse:\n  print((chr(ord(l[-1])+1)))\n", "passed": true, "time": 0.14, "memory": 14116.0, "status": "done"}, {"code": "alpha=\"abcdefghijklmnopqrstuvwxyz\"\nS=input()\nfor i in range(len(alpha)) :\n  if alpha[i] not in S :\n    print(alpha[i])\n    return\nprint(\"None\")", "passed": true, "time": 0.24, "memory": 14216.0, "status": "done"}, {"code": "a=input()\ns=[0]*26\nfor i in range(len(a)):\n    s[ord(a[i])-97]=1\nif sum(s)==26:\n    print(\"None\")\nelse:\n    print(chr(s.index(min(s))+97))", "passed": true, "time": 0.14, "memory": 14200.0, "status": "done"}, {"code": "s = set(input())\n\nfor i in range(26):\n    if chr(i + ord(\"a\")) not in s:\n        print((chr(i + ord(\"a\"))))\n        break\nelse:\n    print(\"None\")\n", "passed": true, "time": 0.29, "memory": 14096.0, "status": "done"}, {"code": "S = list(input())\nS = list(set(S))\n\nans = \"None\"\nfor i in range(97, 123):\n    cnt = chr(i)\n    if not cnt in S:\n        ans = cnt\n        break\n        \nprint(ans)", "passed": true, "time": 0.14, "memory": 14236.0, "status": "done"}, {"code": "alpha = list('abcdefghijklmnopqrstuvwxyz')\ns = input()\nfor i in alpha:\n    if not i in s:\n        print(i)\n        return\nprint('None')", "passed": true, "time": 0.15, "memory": 14216.0, "status": "done"}, {"code": "#-*-coding:utf-8-*-\nimport sys\ninput=sys.stdin.readline\n\ndef main():\n    alpha_data={}\n    S = input()\n    [alpha_data.update({chr(i):0}) for i in range(97, 97+26)]\n    \n    for s in S:\n        if s in alpha_data:\n            tmp=alpha_data[s]\n            tmp+=1\n            alpha_data[s]=tmp\n        else:\n            continue\n    for i,v in alpha_data.items():\n        if v==0:\n            print(i)\n            return\n    print(\"None\")\n\ndef __starting_point():\n    main()\n__starting_point()", "passed": true, "time": 0.15, "memory": 14036.0, "status": "done"}, {"code": "S = str(input())\n\nno_ans = \"None\"\nans_list = []\narufabet = [\"a\", \"b\", \"c\", \"d\", \"e\", \"f\", \"g\", \"h\", \"i\", \"j\", \"k\", \"l\", \"m\", \"n\", \"o\", \"p\", \"q\", \"r\", \"s\", \"t\", \"u\", \"v\", \"w\", \"x\", \"y\", \"z\"]\n\nfor i in range(len(arufabet)):\n    if arufabet[i] not in S:\n        ans_list.append(arufabet[i])\n\nif len(ans_list) == 0:\n    print(no_ans)\nelse:\n    print((ans_list[0]))\n", "passed": true, "time": 0.3, "memory": 14252.0, "status": "done"}, {"code": "S = [s for s in input()]\nli = list(set(S))\nli.sort()\n\nans = 'None'\nfor c in [chr(c) for c in range(97, 123)]:\n    if c not in li:\n        ans = c\n        break\nprint(ans)\n", "passed": true, "time": 0.14, "memory": 14156.0, "status": "done"}, {"code": "S=input()\nfor i in \"abcdefghijklmnopqrstuvwxyz\":\n  if not i in S:\n    print(i)\n    break\nelse:\n  print(\"None\")", "passed": true, "time": 0.14, "memory": 14060.0, "status": "done"}, {"code": "s = input()\nfor i in range(97, 123):\n  if chr(i) not in s:\n    print(chr(i))\n    break\nelse: print(\"None\")", "passed": true, "time": 0.18, "memory": 14196.0, "status": "done"}, {"code": "s = input()\nalpha = [\"a\",\"b\",\"c\",\"d\",\"e\",\"f\",\"g\",\"h\",\"i\",\"j\",\"k\",\"l\",\"m\",\"n\",\"o\",\"p\",\"q\",\"r\",\"s\",\"t\",\"u\",\"v\",\"w\",\"x\",\"y\",\"z\"]\nl = list(set(alpha) - set(s))\nl.sort()\nprint(l[0] if len(l) != 0 else \"None\")", "passed": true, "time": 0.18, "memory": 14396.0, "status": "done"}, {"code": "s = set(input())\nalp = 'abcdefghijklmnopqrstuvwxyz'\nfor a in alp:\n    if a not in s:\n        print(a)\n        return\nprint('None')", "passed": true, "time": 0.24, "memory": 14156.0, "status": "done"}, {"code": "s = input()\ncnt = 0\nfor i in range(26):\n    if chr(97 + i) in s:\n        cnt += 1\n        if cnt == 26:\n            print(\"None\")\n            break\n    else:\n        print(chr(97+i))\n        break", "passed": true, "time": 0.23, "memory": 14336.0, "status": "done"}, {"code": "S = input()\n\na = [True for _ in range(26)]\n\nfor s in S:\n    a[ord(s) - ord(\"a\")] = False\n\nfor i, aa in enumerate(a):\n    if aa:\n        print((chr(i + ord(\"a\"))))\n        return\n\nprint(\"None\")\n", "passed": true, "time": 0.18, "memory": 14260.0, "status": "done"}, {"code": "s=input()\nalphabet='abcdefghijklmnopqrstuvwxyz'\nfor i in alphabet:\n  if i not in s:\n    print(i)\n    return\nprint('None')", "passed": true, "time": 0.14, "memory": 14204.0, "status": "done"}, {"code": "S = list(input())\na = list(\"abcdefghijklmnopqrstuvwxyz\")\nfor i in S:\n    if i in a:\n        a.remove(i)\n\nif a:\n    print(a[0])\nelse:\n    print(\"None\")", "passed": true, "time": 0.24, "memory": 14036.0, "status": "done"}, {"code": "S = list(input())\nL = len(S)\nSlist = set()\nfor i in range(L):\n    Slist.add(ord(S[i]))\nSlist = list(Slist)\nSlist.sort()\nans = \"None\"\nj = 0\nL = len(Slist)\nfor i in range(97, 123):\n    if Slist[j] == i:\n        j += 1\n        if j >= L and i+1 < 123:\n            ans = chr(i+1)\n            break\n        continue\n    ans = chr(i)\n    break\nprint(ans)\n", "passed": true, "time": 0.17, "memory": 14116.0, "status": "done"}, {"code": "#from statistics import median\n#import collections\n#aa = collections.Counter(a) # list to list || .most_common(2)\u3067\u6700\u5927\u306e2\u500b\u3068\u308a\u3060\u305b\u308b\u304a a[0][0]\nfrom math import gcd\nfrom itertools import combinations,permutations,accumulate, product # (string,3) 3\u56de\n#from collections import deque\nfrom collections import deque,defaultdict,Counter\nimport decimal\nimport re\nimport math\nimport bisect\nimport heapq\n#\n#\n#\n# python\u3067\u7121\u7406\u306a\u3068\u304d\u306f\u3001pypy\u3067\u3084\u308b\u3068\u6b63\u89e3\u3059\u308b\u304b\u3082\uff01\uff01\n#\n#\n# my_round_int = lambda x:np.round((x*2 + 1)//2)\n# \u56db\u6368\u4e94\u5165g\n#\n# \u30a4\u30f3\u30c7\u30c3\u30af\u30b9\u7cfb\n# int min_y = max(0, i - 2), max_y = min(h - 1, i + 2);\n# int min_x = max(0, j - 2), max_x = min(w - 1, j + 2);\n#\n#\nimport sys\nsys.setrecursionlimit(10000000)\nmod = 10**9 + 7\n#mod = 9982443453\n#mod = 998244353\nINF = float('inf')\nfrom sys import stdin\nreadline = stdin.readline\ndef readInts():\n  return list(map(int,readline().split()))\ndef readTuples():\n    return tuple(map(int,readline().split()))\ndef I():\n    return int(readline())\ns = input()\nlis = \"abcdefghijklmnopqrstuvwxyz\"\nfor i in range(len(lis)):\n    if lis[i] in s:\n        pass\n    else:\n        print((lis[i]))\n        return\nprint('None')\n", "passed": true, "time": 0.14, "memory": 14240.0, "status": "done"}, {"code": "S=input()\nT=\"abcdefghijklmnopqrstuvwxyz\"\n\nfor c in T:\n    if S.count(c)==0:\n        print(c)\n        break;\nelse:\n    print(\"None\")\n", "passed": true, "time": 0.19, "memory": 14328.0, "status": "done"}, {"code": "S = set(list(input()))\ncf = [chr(ord(\"a\")+i) for i in range(26)]\n\nfor i in range(len(cf)):\n    if cf[i] not in S:\n        print(cf[i])\n        return\nprint(\"None\")", "passed": true, "time": 0.14, "memory": 14284.0, "status": "done"}, {"code": "import string\n\nall_string = set(string.ascii_lowercase)\nS = set(input())\n\nans = sorted(list(all_string - S))\n\nprint(ans[0]) if ans != [] else print(\"None\")", "passed": true, "time": 0.14, "memory": 14212.0, "status": "done"}, {"code": "from collections import Counter\nS = list(input())\nS = Counter(S)\nif len(S) == 26:\n    ans = \"None\"\nelse:\n    A = [chr(i) for i in range(97, 97+26)]\n    for i in A:\n        if i in S:\n            continue\n        ans = i\n        break\nprint(ans)\n", "passed": true, "time": 0.14, "memory": 14316.0, "status": "done"}, {"code": "S=input()\nX=set([s for s in 'abcdefghijklmnopqrstuvwxyz'])\nfor s in S:\n  X.discard(s)\nX=list(X)\nX.sort()\nX.append('None')\nprint(X[0])", "passed": true, "time": 0.14, "memory": 14076.0, "status": "done"}, {"code": "s = input()\nr = set(map(chr, range(97, 97+26)))\nr = r - set(s)\nif len(r) == 0:\n    print('None')\nelse:\n    r = min(r)\n    print(r)", "passed": true, "time": 0.14, "memory": 14184.0, "status": "done"}, {"code": "alphabets = [chr(ord('a') + i) for i in range(26)]\n\nfor a in list(input()):\n    if a in alphabets:\n        alphabets.remove(a)\n\n\nif alphabets:\n    print((alphabets[0]))\nelse:\n    print(\"None\")\n", "passed": true, "time": 0.13, "memory": 14096.0, "status": "done"}, {"code": "from string import ascii_lowercase\n\n\ndef answer(s: str) -> str:\n    not_in_s = list(set(ascii_lowercase) - set(s))\n    if not not_in_s:\n        return 'None'\n    not_in_s.sort()\n\n    return not_in_s[0]\n\n\ndef main():\n    s = input()\n    print(answer(s))\n\n\ndef __starting_point():\n    main()\n__starting_point()", "passed": true, "time": 0.14, "memory": 14280.0, "status": "done"}, {"code": "import sys\nalphabet = \"abcdefghijklmnopqrstuvwxyz\"\ns = set(sorted(input().strip()))\nfor i in alphabet:\n    if not i in s:\n        print(i)\n        return\nprint(\"None\")\n\n\n", "passed": true, "time": 0.14, "memory": 14272.0, "status": "done"}, {"code": "s = str(input())\n\nalp = set(\"abcdefghijklmnopqrstuvwxyz\")\ns = set(s)\n#print(s & alp)\n\nif len(s & alp)==26:\n    ans = \"None\"\nelse:\n    lis = list(alp-s)\n    lis.sort()\n    ans = lis[0]\n\nprint(ans)\n", "passed": true, "time": 0.14, "memory": 14276.0, "status": "done"}, {"code": "print(\"None\" if len(s:=(set(list(\"abcdefghijklmnopqrstuvwxyz\"))-set(input())))==0 else sorted(list(s))[0])", "passed": true, "time": 0.14, "memory": 14072.0, "status": "done"}, {"code": "s = input()\nd_l = [chr(i) for i in range(97, 97+26)]\nans = None\nfor d in d_l:\n    if d not in list(s):\n        ans = d\n        break\nprint(ans)", "passed": true, "time": 0.14, "memory": 14212.0, "status": "done"}, {"code": "print(min(set(map(chr,range(97,123)))-set(input())or['None']))", "passed": true, "time": 0.14, "memory": 14276.0, "status": "done"}, {"code": "s = input()\nalpabet = [chr(x) for x in range(97,123)]\ntry:\n    for a in alpabet:\n        s.index(a)\n    else:\n        print(None)\nexcept ValueError as e:\n    print(a)", "passed": true, "time": 0.21, "memory": 13984.0, "status": "done"}, {"code": "import re\ns = str(input())\nl = list('abcdefghijklmnopqrstuvwxyz')\n\nfor i in l:\n    if not re.search(i,s):\n        print(i)\n        break\nelse:\n    print('None')", "passed": true, "time": 0.14, "memory": 14192.0, "status": "done"}, {"code": "s = input()\nalphabet = \"abcdefghijklmnopqrstuvwxyz\"\n\nfor i in alphabet:\n    if i in s:\n        alphabet = alphabet.replace(i,\"\")\n\nif len(alphabet) == 0:\n    print(\"None\")\nelse:\n    print(alphabet[0])", "passed": true, "time": 0.24, "memory": 14188.0, "status": "done"}, {"code": "s = sorted(set(input()))\na = len(set(s))\ns.append(0)\nfor i in range(26):\n    if s[i] != chr(97+i):\n        print(chr(97+i))\n        break\nelse:\n    print('None')", "passed": true, "time": 0.14, "memory": 14292.0, "status": "done"}, {"code": "import string\nS = set(input())\nfor i in range(97, 123):\n  s = chr(i)\n  if s not in S:\n    print(s)\n    break\nelse:\n  print(None)", "passed": true, "time": 0.15, "memory": 14188.0, "status": "done"}, {"code": "S = input()\nfor c in \"abcdefghijklmnopqrstuvwxyz\":\n    if c not in S:\n        print(c)\n        break\nelse:\n    print(\"None\")\n", "passed": true, "time": 0.24, "memory": 14160.0, "status": "done"}, {"code": "s = sorted(list(set(input())))\n\nfor i in range(26):\n    if chr(ord(\"a\") + i) not in s:\n         print(chr(ord(\"a\") + i))\n         break\nelse:\n    print(\"None\")", "passed": true, "time": 0.18, "memory": 14100.0, "status": "done"}, {"code": "s = input()\ns = list(s)\ns = set(s)\nl = [chr(ord(\"a\")+i) for i in range(26)]\nans = 'None'\nfor i in l:\n    if i not in s:\n        ans = i\n        break\nprint(ans)", "passed": true, "time": 0.28, "memory": 14168.0, "status": "done"}, {"code": "s = input()\nalphabet = 'abcdefghijklmnopqrstuvwxyz'\n\nfor i in alphabet:\n  if i not in s:\n    print(i)\n    return\nprint('None')", "passed": true, "time": 0.14, "memory": 14048.0, "status": "done"}, {"code": "# -*- coding:utf-8 -*-\nS = input()\n\nalpahbet = \"abcdefghijklmnopqrstuvwxyz\"\nstring_list = []\nans = \"None\"\nfor s in S:\n    if s not in string_list:\n        string_list.append(s)\n\nfor a in alpahbet:\n    if a not in string_list:\n        ans = a\n        break\n\nprint(ans)", "passed": true, "time": 0.14, "memory": 14340.0, "status": "done"}, {"code": "s=input()\n\nfor i in \"abcdefghijklmnopqrstuvwxyz\":\n  if i not in s:\n    print(i)\n    break\nelse:\n  print(\"None\")", "passed": true, "time": 0.14, "memory": 14060.0, "status": "done"}, {"code": "S = list(input())\nalpha = 'abcdefghijklmnopqrstuvwxyz'\n\nfor a in alpha:\n  if S.count(a) == 0:\n    print(a)\n    break\nelse:\n  print(\"None\")", "passed": true, "time": 0.14, "memory": 14308.0, "status": "done"}, {"code": "s = input()\nc = \"a\"\nwhile c <= \"z\":\n    if s.find(c) == -1:\n        print(c)\n        break\n    if c == \"z\":\n        print(\"None\")\n    c = chr(ord(c) + 1)\n\n", "passed": true, "time": 0.14, "memory": 14232.0, "status": "done"}, {"code": "import string\nL = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']\nS = set(input())\nfor i in L:\n  if i not in S:\n    print(i)\n    break\nelse:\n  print(None)", "passed": true, "time": 0.14, "memory": 14352.0, "status": "done"}]