n, k = map(int,input().split())
p = list(map(int,input().split()))
mu = [(a+1)/2 for a in p]
maxsum = sum(mu[:k])
now = maxsum
left = 0
right = k
while right<n:
    now = now+mu[right]-mu[left]
    if maxsum<now:
        maxsum = now
    left += 1
    right += 1
print(maxsum) 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 (a+1)/2 for a in p for a in p a p for a in p (a+1)/2 a+1 a 1 2 mu = [(a+1)/2 for a in p] mu [(a+1)/2 for a in p] maxsum = sum(mu[:k]) maxsum sum(mu[:k]) sum mu[:k] mu :k k now = maxsum now maxsum left = 0 left 0 right = k right k while right<n:
    now = now+mu[right]-mu[left]
    if maxsum<now:
        maxsum = now
    left += 1
    right += 1 right<n right n now = now+mu[right]-mu[left] now now+mu[right]-mu[left] now+mu[right] now mu[right] mu right mu[left] mu left if maxsum<now:
        maxsum = now
     maxsum<now maxsum now maxsum = now maxsum now left += 1 left 1 right += 1 right 1 print(maxsum) print maxsum maxsum = now now maxsum left = 0 0 left left += 1 1 left maxsum = sum(mu[:k]) sum(mu[:k]) maxsum n, k = map(int,input().split()) map(int,input().split()) n right += 1 1 right mu = [(a+1)/2 for a in p] [(a+1)/2 for a in p] mu right = k k right p = list(map(int,input().split())) list(map(int,input().split())) p now = now+mu[right]-mu[left] now+mu[right]-mu[left] now k = map(int,input().split()) map(int,input().split()) k now = maxsum maxsum now
n, k = map(int,input().split())
p = list(map(int,input().split()))
mu = [(a+1)/2 for a in p]
maxsum = sum(mu[:k])
now = maxsum
left = 0
right = k
while right<n:
    now = now+mu[right]-mu[left]
    if maxsum<now:
        maxsum = now
    left += 1
    right += 1
print(maxsum)