1283_F. DIY Garland_17112
Polycarp has decided to decorate his room because the New Year is soon. One of the main decorations that Polycarp will install is the garland he is going to solder himself.

Simple garlands consisting of several lamps connected by one wire are too boring for Polycarp. He is going to solder a garland consisting of n lamps and n - 1 wires. Exactly one lamp will be connected to power grid, and power will be transmitted from it to other lamps by the wires. Each wire connectes exactly two lamps; one lamp is called the main lamp for this wire (the one that gets power from some other wire and transmits it to this wire), the other one is called the auxiliary lamp (the one that gets power from this wire). Obviously, each lamp has at most one wire that brings power to it (and this lamp is the auxiliary lamp for this wire, and the main lamp for all other wires connected directly to it).

Each lamp has a brightness value associated with it, the i-th lamp has brightness 2^i. We define the importance of the wire as the sum of brightness values over all lamps that become disconnected from the grid if the wire is cut (and all other wires are still working).

Polycarp has drawn the scheme of the garland he wants to make (the scheme depicts all n lamp and n - 1 wires, and the lamp that will be connected directly to the grid is marked; the wires are placed in such a way that the power can be transmitted to each lamp). After that, Polycarp calculated the importance of each wire, enumerated them from 1 to n - 1 in descending order of their importance, and then wrote the index of the main lamp for each wire (in the order from the first wire to the last one).

The following day Polycarp bought all required components of the garland and decided to solder it — but he could not find the scheme. Fortunately, Polycarp found the list of indices of main lamps for all wires. Can you help him restore the original scheme?

Input

The first line contains one integer n (2 ≤ n ≤ 2 ⋅ 10^5) — the number of lamps.

The second line contains n - 1 integers a_1, a_2, ..., a_{n - 1} (1 ≤ a_i ≤ n), where a_i is the index of the main lamp for the i-th wire (wires are numbered in descending order of importance).

Output

If it is impossible to restore the original scheme, print one integer -1.

Otherwise print the scheme as follows. In the first line, print one integer k (1 ≤ k ≤ n) — the index of the lamp that is connected to the power grid. Then print n - 1 lines, each containing two integers x_i and y_i (1 ≤ x_i, y_i ≤ n, x_i ≠ y_i) — the indices of the lamps connected by some wire. The descriptions of the wires (and the lamps connected by a wire) can be printed in any order. The printed description must correspond to a scheme of a garland such that Polycarp could have written the list a_1, a_2, ..., a_{n - 1} from it. If there are multiple such schemes, output any of them.

Example

Input


6
3 6 3 1 5


Output


3
6 3
6 5
1 3
1 4
5 2

Note

The scheme for the first example (R denotes the lamp connected to the grid, the numbers on wires are their importance values):

<image>
# Enter your code here. Read input from STDIN. Print output to STDOUT# ===============================================================================================
# importing some useful libraries.
from __future__ import division, print_function
from fractions import Fraction
import sys
import os
from io import BytesIO, IOBase
from itertools import *
import bisect
from heapq import *
from math import ceil, floor
from copy import *
from collections import deque, defaultdict
from collections import Counter as counter  # Counter(list)  return a dict with {key: count}
from itertools import combinations  # if a = [1,2,3] then print(list(comb(a,2))) -----> [(1, 2), (1, 3), (2, 3)]
from itertools import permutations as permutate
from bisect import bisect_left as bl
from operator import *
# If the element is already present in the list,

# the left most position where element has to be inserted is returned.
from bisect import bisect_right as br
from bisect import bisect

# If the element is already present in the list,
# the right most position where element has to be inserted is returned

# ==============================================================================================
# fast I/O region

BUFSIZE = 8192
from sys import stderr


class FastIO(IOBase):
    newlines = 0

    def __init__(self, file):
        self._fd = file.fileno()
        self.buffer = BytesIO()
        self.writable = "A" in file.mode or "r" not in file.mode
        self.write = self.buffer.write if self.writable else None

    def read(self):
        while True:
            b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
            if not b:
                break
            ptr = self.buffer.tell()
            self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
        self.newlines = 0
        return self.buffer.read()

    def readline(self):
        while self.newlines == 0:
            b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
            self.newlines = b.count(b"\n") + (not b)
            ptr = self.buffer.tell()
            self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
        self.newlines -= 1
        return self.buffer.readline()

    def flush(self):
        if self.writable:
            os.write(self._fd, self.buffer.getvalue())
            self.buffer.truncate(0), self.buffer.seek(0)


class IOWrapper(IOBase):
    def __init__(self, file):
        self.buffer = FastIO(file)
        self.flush = self.buffer.flush
        self.writable = self.buffer.writable
        self.write = lambda s: self.buffer.write(s.encode("ascii"))
        self.read = lambda: self.buffer.read().decode("ascii")
        self.readline = lambda: self.buffer.readline().decode("ascii")


def print(*args, **kwargs):
    """Prints the values to a stream, or to sys.stdout by default."""
    sep, file = kwargs.pop("sep", " "), kwargs.pop("file", sys.stdout)
    at_start = True
    for A in args:
        if not at_start:
            file.write(sep)
        file.write(str(A))
        at_start = False
    file.write(kwargs.pop("end", "\n"))
    if kwargs.pop("flush", False):
        file.flush()


if sys.version_info[0] < 3:
    sys.stdin, sys.stdout = FastIO(sys.stdin), FastIO(sys.stdout)
else:
    sys.stdin, sys.stdout = IOWrapper(sys.stdin), IOWrapper(sys.stdout)

# inp = lambda: sys.stdin.readline().rstrip("\r\n")

# ===============================================================================================
### START ITERATE RECURSION ###
from types import GeneratorType


def iterative(f, stack=[]):
    def wrapped_func(*args, **kwargs):
        if stack: return f(*args, **kwargs)
        to = f(*args, **kwargs)
        while True:
            if type(to) is GeneratorType:
                stack.append(to)
                to = next(to)
                continue
            stack.pop()
            if not stack: break
            to = stack[-1].send(to)
        return to

    return wrapped_func


#### END ITERATE RECURSION ####
###########################
# Sorted list
class SortedList:
    def __init__(self, iterable=[], _load=200):
        """Initialize sorted list instance."""
        values = sorted(iterable)
        self._len = _len = len(values)
        self._load = _load
        self._lists = _lists = [values[start:start + _load] for start in range(0, _len, _load)]
        self._list_lens = [len(_list) for _list in _lists]
        self._mins = [_list[0] for _list in _lists]
        self._fen_tree = []
        self._rebuild = True

    def _fen_build(self):
        """Build a fenwick tree instance."""
        self._fen_tree[:] = self._list_lens
        _fen_tree = self._fen_tree
        for start in range(len(_fen_tree)):
            if start | start + 1 < len(_fen_tree):
                _fen_tree[start | start + 1] += _fen_tree[start]
        self._rebuild = False

    def _fen_update(self, index, value):
        """Update `fen_tree[index] += value`."""
        if not self._rebuild:
            _fen_tree = self._fen_tree
            while index < len(_fen_tree):
                _fen_tree[index] += value
                index |= index + 1

    def _fen_query(self, end):
        """Return `sum(_fen_tree[:end])`."""
        if self._rebuild:
            self._fen_build()

        _fen_tree = self._fen_tree
        A = 0
        while end:
            A += _fen_tree[end - 1]
            end &= end - 1
        return A

    def _fen_findkth(self, k):
        """Return a pair of (the largest `idx` such that `sum(_fen_tree[:idx]) <= k`, `k - sum(_fen_tree[:idx])`)."""
        _list_lens = self._list_lens
        if k < _list_lens[0]:
            return 0, k
        if k >= self._len - _list_lens[-1]:
            return len(_list_lens) - 1, k + _list_lens[-1] - self._len
        if self._rebuild:
            self._fen_build()

        _fen_tree = self._fen_tree
        idx = -1
        for d in reversed(range(len(_fen_tree).bit_length())):
            right_idx = idx + (1 << d)
            if right_idx < len(_fen_tree) and k >= _fen_tree[right_idx]:
                idx = right_idx
                k -= _fen_tree[idx]
        return idx + 1, k

    def _delete(self, pos, idx):
        """Delete value at the given `(pos, idx)`."""
        _lists = self._lists
        _mins = self._mins
        _list_lens = self._list_lens

        self._len -= 1
        self._fen_update(pos, -1)
        del _lists[pos][idx]
        _list_lens[pos] -= 1

        if _list_lens[pos]:
            _mins[pos] = _lists[pos][0]
        else:
            del _lists[pos]
            del _list_lens[pos]
            del _mins[pos]
            self._rebuild = True

    def _loc_left(self, value):
        """Return an index pair that corresponds to the first position of `value` in the sorted list."""
        if not self._len:
            return 0, 0

        _lists = self._lists
        _mins = self._mins

        lo, pos = -1, len(_lists) - 1
        while lo + 1 < pos:
            mi = (lo + pos) >> 1
            if value <= _mins[mi]:
                pos = mi
            else:
                lo = mi

        if pos and value <= _lists[pos - 1][-1]:
            pos -= 1

        _list = _lists[pos]
        lo, idx = -1, len(_list)
        while lo + 1 < idx:
            mi = (lo + idx) >> 1
            if value <= _list[mi]:
                idx = mi
            else:
                lo = mi

        return pos, idx

    def _loc_right(self, value):
        """Return an index pair that corresponds to the last position of `value` in the sorted list."""
        if not self._len:
            return 0, 0

        _lists = self._lists
        _mins = self._mins

        pos, hi = 0, len(_lists)
        while pos + 1 < hi:
            mi = (pos + hi) >> 1
            if value < _mins[mi]:
                hi = mi
            else:
                pos = mi

        _list = _lists[pos]
        lo, idx = -1, len(_list)
        while lo + 1 < idx:
            mi = (lo + idx) >> 1
            if value < _list[mi]:
                idx = mi
            else:
                lo = mi

        return pos, idx

    def add(self, value):
        """Add `value` to sorted list."""
        _load = self._load
        _lists = self._lists
        _mins = self._mins
        _list_lens = self._list_lens

        self._len += 1
        if _lists:
            pos, idx = self._loc_right(value)
            self._fen_update(pos, 1)
            _list = _lists[pos]
            _list.insert(idx, value)
            _list_lens[pos] += 1
            _mins[pos] = _list[0]
            if _load + _load < len(_list):
                _lists.insert(pos + 1, _list[_load:])
                _list_lens.insert(pos + 1, len(_list) - _load)
                _mins.insert(pos + 1, _list[_load])
                _list_lens[pos] = _load
                del _list[_load:]
                self._rebuild = True
        else:
            _lists.append([value])
            _mins.append(value)
            _list_lens.append(1)
            self._rebuild = True

    def discard(self, value):
        """Remove `value` from sorted list if it is a member."""
        _lists = self._lists
        if _lists:
            pos, idx = self._loc_right(value)
            if idx and _lists[pos][idx - 1] == value:
                self._delete(pos, idx - 1)

    def remove(self, value):
        """Remove `value` from sorted list; `value` must be a member."""
        _len = self._len
        self.discard(value)
        if _len == self._len:
            raise ValueError('{0!r} not in list'.format(value))

    def pop(self, index=-1):
        """Remove and return value at `index` in sorted list."""
        pos, idx = self._fen_findkth(self._len + index if index < 0 else index)
        value = self._lists[pos][idx]
        self._delete(pos, idx)
        return value

    def bisect_left(self, value):
        """Return the first index to insert `value` in the sorted list."""
        pos, idx = self._loc_left(value)
        return self._fen_query(pos) + idx

    def bisect_right(self, value):
        """Return the last index to insert `value` in the sorted list."""
        pos, idx = self._loc_right(value)
        return self._fen_query(pos) + idx

    def count(self, value):
        """Return number of occurrences of `value` in the sorted list."""
        return self.bisect_right(value) - self.bisect_left(value)

    def __len__(self):
        """Return the size of the sorted list."""
        return self._len

    def __getitem__(self, index):
        """Lookup value at `index` in sorted list."""
        pos, idx = self._fen_findkth(self._len + index if index < 0 else index)
        return self._lists[pos][idx]

    def __delitem__(self, index):
        """Remove value at `index` from sorted list."""
        pos, idx = self._fen_findkth(self._len + index if index < 0 else index)
        self._delete(pos, idx)

    def __contains__(self, value):
        """Return true if `value` is an element of the sorted list."""
        _lists = self._lists
        if _lists:
            pos, idx = self._loc_left(value)
            return idx < len(_lists[pos]) and _lists[pos][idx] == value
        return False

    def __iter__(self):
        """Return an iterator over the sorted list."""
        return (value for _list in self._lists for value in _list)

    def __reversed__(self):
        """Return a reverse iterator over the sorted list."""
        return (value for _list in reversed(self._lists) for value in reversed(_list))

    def __repr__(self):
        """Return string representation of sorted list."""
        return 'SortedList({0})'.format(list(self))


# ===============================================================================================
# some shortcuts

mod = 1000000007


def testcase(t):
    for p in range(t):
        solve()


def pow(A, B, p):
    res = 1  # Initialize result
    A = A % p  # Update A if it is more , than or equal to p
    if (A == 0):
        return 0
    while (B > 0):
        if ((B & 1) == 1):  # If B is odd, multiply, A with result
            res = (res * A) % p

        B = B >> 1  # B = B/2
        A = (A * A) % p
    return res


from functools import reduce


def factors(n):
    return set(reduce(list.__add__,
                      ([start, n // start] for start in range(1, int(n ** 0.5) + 1) if n % start == 0)))


def gcd(a, b):
    if a == b: return a
    while b > 0: a, b = b, a % b
    return a


# discrete binary search
# minimise:
# def search():
#     l = 0
#     r = 10 ** 15
#
#     for start in range(200):
#         if isvalid(l):
#             return l
#         if l == r:
#             return l
#         m = (l + r) // 2
#         if isvalid(m) and not isvalid(m - 1):
#             return m
#         if isvalid(m):
#             r = m + 1
#         else:
#             l = m
#     return m

# maximise:
# def search():
#     l = 0
#     r = 10 ** 15
#
#     for start in range(200):
#         # print(l,r)
#         if isvalid(r):
#             return r
#         if l == r:
#             return l
#         m = (l + r) // 2
#         if isvalid(m) and not isvalid(m + 1):
#             return m
#         if isvalid(m):
#             l = m
#         else:
#             r = m - 1
#     return m


##############Find sum of product of subsets of size k in a array
# ar=[0,1,2,3]
# k=3
# n=len(ar)-1
# dp=[0]*(n+1)
# dp[0]=1
# for pos in range(1,n+1):
#     dp[pos]=0
#     l=max(1,k+pos-n-1)
#     for j in range(min(pos,k),l-1,-1):
#         dp[j]=dp[j]+ar[pos]*dp[j-1]
# print(dp[k])

def prefix_sum(ar):  # [1,2,3,4]->[1,3,6,10]
    return list(accumulate(ar))


def suffix_sum(ar):  # [1,2,3,4]->[10,9,7,4]
    return list(accumulate(ar[::-1]))[::-1]


def N():
    return int(inp())


dx = [0, 0, 1, -1]
dy = [1, -1, 0, 0]


def YES():
    print("YES")


def NO():
    print("NO")


def Yes():
    print("Yes")


def No():
    print("No")


# =========================================================================================
from collections import defaultdict


def numberOfSetBits(start):
    start = start - ((start >> 1) & 0x55555555)
    start = (start & 0x33333333) + ((start >> 2) & 0x33333333)
    return (((start + (start >> 4) & 0xF0F0F0F) * 0x1010101) & 0xffffffff) >> 24


class MergeFind:
    def __init__(self, n):
        self.parent = list(range(n))
        self.size = [1] * n
        self.num_sets = n
        # self.lista = [[_] for _ in range(n)]

    def find(self, a):
        to_update = []
        while a != self.parent[a]:
            to_update.append(a)
            a = self.parent[a]
        for b in to_update:
            self.parent[b] = a
        return self.parent[a]

    def merge(self, a, b):
        a = self.find(a)
        b = self.find(b)
        if a == b:
            return
        if self.size[a] < self.size[b]:
            a, b = b, a
        self.num_sets -= 1
        self.parent[b] = a
        self.size[a] += self.size[b]
        # self.lista[a] += self.lista[b]
        # self.lista[b] = []

    def set_size(self, a):
        return self.size[self.find(a)]

    def __len__(self):
        return self.num_sets


def lcm(a, b):
    return abs((a // gcd(a, b)) * b)


# #
# to find factorial and ncr
# tot = 100005
# mod = 10**9 + 7
# fac = [1, 1]
# finv = [1, 1]
# inv = [0, 1]
#
# for start in range(2, tot + 1):
#     fac.append((fac[-1] * start) % mod)
#     inv.append(mod - (inv[mod % start] * (mod // start) % mod))
#     finv.append(finv[-1] * inv[-1] % mod)


def comb(n, r):
    if n < r:
        return 0
    else:
        return fac[n] * (finv[r] * finv[n - r] % mod) % mod


def inp(): return sys.stdin.readline().rstrip("\r\n")  # for fast input


def out(var): sys.stdout.write(str(var))  # for fast output, always take string


def lis(): return list(map(int, inp().split()))


def stringlis(): return list(map(str, inp().split()))


def sep(): return map(int, inp().split())


def strsep(): return map(str, inp().split())


def fsep(): return map(float, inp().split())


def nextline(): out("\n")  # as stdout.write always print sring.


def arr1d(n, v):
    return [v] * n


def arr2d(n, m, v):
    return [[v] * m for _ in range(n)]


def arr3d(n, m, p, v):
    return [[[v] * p for _ in range(m)] for start in range(n)]


def ceil(a, b):
    return (a + b - 1) // b


# co-ordinate compression
# ma={s:idx for idx,s in enumerate(sorted(set(l+r)))}

# mxn=100005
# lrg=[0]*mxn
# for start in range(2,mxn-3):
#     if (lrg[start]==0):
#         for j in range(start,mxn-3,start):
#             lrg[j]=start


def solve():
    n=N()
    ar=lis()
    ans=[]
    come=[0]*(n+1)
    last=n
    print(ar[0])
    for i in range(n-1):
        temp=ar[i]
        come[temp]=1
        if(come[last]):
            while(come[last]):
                last-=1
        if(i+1>=n-1 or come[ar[i+1]]==1):
            ans.append((temp, last))
            come[last] = 1
            last-=1

        else:
            nxt=ar[i+1]
            ans.append((temp, nxt))
            come[nxt] = 1
        # print(temp,ans,last,come)

    for i in ans:
        print(*i)












solve()
#testcase(N())