import sys

import numpy as np

read = sys.stdin.buffer.read

readline = sys.stdin.buffer.readline

readlines = sys.stdin.buffer.readlines



n, k = list(map(int, readline().split()))

p = list(map(int, readline().split()))



tmp = [(i+1)/2 for i in p]

cs = list(np.cumsum(tmp))



if n == k:

    print((cs[-1]))

    exit()

ans = 0

for i in range(n - k):

    ans = max(ans, cs[i + k] - cs[i])

print(ans)
import sys

# import numpy as np

read = sys.stdin.buffer.read

readline = sys.stdin.buffer.readline

readlines = sys.stdin.buffer.readlines



n, k = list(map(int, readline().split()))

p = list(map(int, readline().split()))



tmp = [(i+1)/2 for i in p]

# cs = list(np.cumsum(tmp))



cs = [0]*n

cs[0] = tmp[0]

for i in range(len(tmp)-1):

    cs[i + 1] = cs[i] + tmp[i + 1]



if n == k:

    print((cs[-1]))

    exit()

ans = 0

for i in range(n - k):

    ans = max(ans, cs[i + k] - cs[i])

print(ans)
import sys
import numpy as np

read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines
n, k = list(map(int, readline().split()))
p = list(map(int, readline().split()))
tmp = [(i + 1) / 2 for i in p]
cs = list(np.cumsum(tmp))
if n == k:
    print((cs[-1]))
    exit()
ans = 0
for i in range(n - k):
    ans = max(ans, cs[i + k] - cs[i])
print(ans)

import sys

# import numpy as np
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines
n, k = list(map(int, readline().split()))
p = list(map(int, readline().split()))
tmp = [(i + 1) / 2 for i in p]
# cs = list(np.cumsum(tmp))
cs = [0] * n
cs[0] = tmp[0]
for i in range(len(tmp) - 1):
    cs[i + 1] = cs[i] + tmp[i + 1]
if n == k:
    print((cs[-1]))
    exit()
ans = 0
for i in range(n - k):
    ans = max(ans, cs[i + k] - cs[i])
print(ans)

-import numpy as np +# import numpy as np -cs = list(np.cumsum(tmp)) +# cs = list(np.cumsum(tmp)) +cs = [0] * n +cs[0] = tmp[0] +for i in range(len(tmp) - 1): +    cs[i + 1] = cs[i] + tmp[i + 1]