N, K = map(int, input().split())
P = list(map(int, input().split()))
Pe = [(i+1)/2 for i in P]

maxsum = -1
cur = sum(Pe[0:K])
maxsum = cur
for i in range(1, N-K+1):
    cur = cur - Pe[i-1] + Pe[i+K-1]
    if cur > maxsum:
        maxsum = cur

print(maxsum)