n,k=map(int,input().split())
p=list(map(int,input().split()))
q=[(1+pp)/2 for pp in p]
ans=0
c=[0]
for qq in q:
      c.append(c[-1]+qq)
ans=0

for qq in range(k,1+n):
  ans=max(ans,c[qq]-c[qq-k])

print(ans)
