#include <iostream>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <string>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <stdio.h>
using namespace std;
#define int long long
int MOD = 1000000007;
vector<int> A;
vector<double> B;
signed main() {
	cin.tie(0);
	ios::sync_with_stdio(false);
	int N, K;
	cin >> N >> K;
	A.resize(2 * N);
	B.resize(2 * N + 1);
	string S;
	cin >> S;

	for (int i = 0; i < N; i++) {
		A[i] = (int)(S[i]-'0');
		A[i + N] = A[i];
	}
	A.push_back(0);
	double u = 1;
	double d = 0;
	double m;
	double mx;
	bool f;
	while (u-d>1e-8) {
		
		m = (u + d) / 2;
		mx = 0;
		f = false;
		B[0] = m - A[0];
		for (int i = 1; i < 2 * N + 1; i++) {
			B[i] = B[i - 1] - A[i];
			B[i] += m;
		}
		for (int i = K - 1; i < 2 * N + 1; i++) {
			
			if (i >= K) {
				mx = max(mx, B[i - K]);
			}
			if (B[i] < mx) {
				f = true;
				break;
			}
		}
		if (f) {
			d = m;
		}
		else {
			u = m;
		}
	}
	cout << setprecision(10) << m << endl;
}