n,m = map(int,input().split())
myList = list(map(int,input().split()))
max = 0
for i in range(0,n - m + 1):
     s = 0
     for j in range(i,i + m):
          s = s + myList[j]
     if s / m > max:
          max = s / m
print("%.2f"%(max))
/**************************************************************
	Problem: 1165
	User: admin
	Language: Python
	Result: Accepted
	Time:98 ms
	Memory:34480 kb
****************************************************************/