#!/usr/bin/env python
# -*- coding: utf-8 -*-
#
# FileName: 	D
# CreatedDate:  2020-07-29 16:20:16 +0900
# LastModified: 2020-07-29 16:30:22 +0900
#


import os
import sys
# import numpy as np
# import pandas as pd
from itertools import accumulate


def main():
    n, k = map(int, input().split())
    p = list(map(int, input().split()))
    s = [(x+1)/2 for x in p]
#    print(s)
    s = list(accumulate(s))
    s.insert(0, 0)
    ans = 0
#    print(s)
    for i in range(n-k+1):
        ans = max(ans, s[i+k]-s[i])
    print(ans)



if __name__ == "__main__":
    main() import os os import sys sys from itertools import accumulate def main():
    n, k = map(int, input().split())
    p = list(map(int, input().split()))
    s = [(x+1)/2 for x in p]
#    print(s)
    s = list(accumulate(s))
    s.insert(0, 0)
    ans = 0
#    print(s)
    for i in range(n-k+1):
        ans = max(ans, s[i+k]-s[i])
    print(ans) main n, k = map(int, input().split()) n map(int, input().split()) map int input().split() ().split () input split k p = list(map(int, input().split())) p list(map(int, input().split())) list map(int, input().split()) map int input().split() ().split () input split (x+1)/2 for x in p for x in p x p for x in p (x+1)/2 x+1 x 1 2 s = [(x+1)/2 for x in p] s [(x+1)/2 for x in p] s = list(accumulate(s)) s list(accumulate(s)) list accumulate(s) accumulate s s.insert(0, 0) s.insert s insert 0 0 ans = 0 ans 0 for i in range(n-k+1):
        ans = max(ans, s[i+k]-s[i])
     i range(n-k+1) range n-k+1 n-k n k 1 ans = max(ans, s[i+k]-s[i]) ans max(ans, s[i+k]-s[i]) max ans s[i+k]-s[i] s[i+k] s i+k i k s[i] s i print(ans) print ans if __name__ == "__main__":
    main() __name__ == "__main__" __name__ "__main__" main() main def main():
    n, k = map(int, input().split())
    p = list(map(int, input().split()))
    s = [(x+1)/2 for x in p]
#    print(s)
    s = list(accumulate(s))
    s.insert(0, 0)
    ans = 0
#    print(s)
    for i in range(n-k+1):
        ans = max(ans, s[i+k]-s[i])
    print(ans) def main():
    n, k = map(int, input().split())
    p = list(map(int, input().split()))
    s = [(x+1)/2 for x in p]
#    print(s)
    s = list(accumulate(s))
    s.insert(0, 0)
    ans = 0
#    print(s)
    for i in range(n-k+1):
        ans = max(ans, s[i+k]-s[i])
    print(ans) main
#!/usr/bin/env python
# -*- coding: utf-8 -*-
#
# FileName: 	D
# CreatedDate:  2020-07-29 16:20:16 +0900
# LastModified: 2020-07-29 16:30:22 +0900
#


import os
import sys
# import numpy as np
# import pandas as pd
from itertools import accumulate


def main():
    n, k = map(int, input().split())
    p = list(map(int, input().split()))
    s = [(x+1)/2 for x in p]
#    print(s)
    s = list(accumulate(s))
    s.insert(0, 0)
    ans = 0
#    print(s)
    for i in range(n-k+1):
        ans = max(ans, s[i+k]-s[i])
    print(ans)



if __name__ == "__main__":
    main()
