import sys
from collections import deque

read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines

N, K = map(int, readline().split())
A = tuple(map(float, read().rstrip().decode()))
A = A + A

def test(x):
    Acum = [0.0] * (N+N+1)
    for i in range(N+N):
        Acum[i+1] = Acum[i] + A[i] - x
    q = deque([0])
    for k in range(K, N+N+1):
        while q[0] < k - N:
            q.popleft()
        if Acum[q[0]] < Acum[k]:
            return True
        i = k - K + 1
        x = Acum[i]
        while q and Acum[q[-1]] > x:
            q.pop()
        q.append(i)
    return False

vals = [0, 1]
for _ in range(22):
    x = sum(vals) / 2
    vals[1 - test(x)] = x

print(vals[0])