def main():

    n, k = map(int, input().split())
    p = list(map(int, input().split()))

    ex = [0]
    for x in p:
        ex.append(ex[-1] + (x+1)/2)

    ans = 0
    for i in range(k, n+1):
        ans = max(ans, ex[i] - ex[i-k])

    print(ans)

main() def main():

    n, k = map(int, input().split())
    p = list(map(int, input().split()))

    ex = [0]
    for x in p:
        ex.append(ex[-1] + (x+1)/2)

    ans = 0
    for i in range(k, n+1):
        ans = max(ans, ex[i] - ex[i-k])

    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 ex = [0] ex [0] 0 for x in p:
        ex.append(ex[-1] + (x+1)/2)

     x p ex.append(ex[-1] + (x+1)/2) ex.append ex append ex[-1] + (x+1)/2 ex[-1] ex -1 (x+1)/2 x+1 x 1 2 ans = 0 ans 0 for i in range(k, n+1):
        ans = max(ans, ex[i] - ex[i-k])

     i range(k, n+1) range k n+1 n 1 ans = max(ans, ex[i] - ex[i-k]) ans max(ans, ex[i] - ex[i-k]) max ans ex[i] - ex[i-k] ex[i] ex i ex[i-k] ex i-k i k print(ans) print ans main() main def main():

    n, k = map(int, input().split())
    p = list(map(int, input().split()))

    ex = [0]
    for x in p:
        ex.append(ex[-1] + (x+1)/2)

    ans = 0
    for i in range(k, n+1):
        ans = max(ans, ex[i] - ex[i-k])

    print(ans) def main():

    n, k = map(int, input().split())
    p = list(map(int, input().split()))

    ex = [0]
    for x in p:
        ex.append(ex[-1] + (x+1)/2)

    ans = 0
    for i in range(k, n+1):
        ans = max(ans, ex[i] - ex[i-k])

    print(ans) main
def main():

    n, k = map(int, input().split())
    p = list(map(int, input().split()))

    ex = [0]
    for x in p:
        ex.append(ex[-1] + (x+1)/2)

    ans = 0
    for i in range(k, n+1):
        ans = max(ans, ex[i] - ex[i-k])

    print(ans)

main()