#include <cstdio>
#include <string>
#include <algorithm>
using namespace std;
int N, M, deq[1000009]; double vs[1000009]; char s[500009];
bool solve(double x) {
	int ps = 0;
	for (int i = 1; i <= 2 * N; i++) {
		ps += s[(i - 1) % N] - 48;
		vs[i] = ps - x * i;
	}
	int l = 0, r = 0;
	for (int i = M; i < N; i++) {
		while (l < r && vs[deq[r - 1]] < vs[i]) r--;
		deq[r++] = i;
	}
	for (int i = 0; i < N; i++) {
		while (l < r && vs[deq[r - 1]] < vs[i + N]) r--;
		deq[r++] = i + N;
		if (vs[deq[l]] >= vs[i] - 1.0e-12) return true;
		if (deq[l] == i + M) l++;
	}
	return false;
}
int main() {
	scanf("%d %d", &N, &M);
	scanf("%s", s);
	double l = 0.0, r = 1.0;
	for (int i = 0; i < 40; i++) {
		double m = (l + r) * 0.5;
		if (solve(m)) l = m;
		else r = m;
	}
	printf("%.12lf\n", l);
	return 0;
}