import sys
from collections import deque

def main():
    N, K = map(int, sys.stdin.readline().split())
    s = sys.stdin.readline().strip()
    A = [int(c) for c in s]
    duplicated = A + A

    low = 0.0
    high = 1.0
    eps = 1e-10
    for _ in range(100):
        mid = (low + high) / 2
        if check(duplicated, N, K, mid, eps):
            low = mid
        else:
            high = mid
    print("{0:.15f}".format(low))

def check(duplicated, N, K, x, eps):
    n = len(duplicated)
    prefix = [0.0] * (n + 1)
    for i in range(1, n+1):
        prefix[i] = prefix[i-1] + (duplicated[i-1] - x)
    dq = deque()
    found = False
    for i in range(1, n+1):
        if i < K:
            continue
        window_start = max(0, i - N)
        window_end = i - K
        if window_end < 0:
            continue
        j = window_end
        # Add j to deque maintaining increasing order of prefix[j]
        while dq and prefix[j] <= prefix[dq[-1]]:
            dq.pop()
        dq.append(j)
        # Remove elements from the front that are out of the window
        while dq and dq[0] < window_start:
            dq.popleft()
        # Check if current sum is non-negative
        if dq:
            current = prefix[i] - prefix[dq[0]]
            if current >= -eps:
                found = True
                break
    return found

if __name__ == '__main__':
    main()