#include <iostream>
#include <iomanip>
using namespace std;
 
int main(){
	int N, K;
	string jewelry;
	cin >> N >> K >> jewelry;
	int blue_gem = 0;
	for(int i = 0; i < K; i++){
		blue_gem += jewelry[i] - '0';
	}
	double ans = 0;
	bool if_retrieval[N];
	for(int i = 0; i < N; i++){
		if_retrieval[i] = true;
	}
	for(int i = 0; i < N; i++){
		if(!if_retrieval[i]){
			continue;
		}
		blue_gem -= jewelry[(i - 1 + N) % N] - '0';
		blue_gem += jewelry[(i + K - 1 + N) % N] - '0';
		int blue_gem_plus = 0;
		for(int j = 1; j + K - 1 < N; j++){
			if(jewelry[(i - j + N) % N] == '0'){
				break;
			}
			blue_gem_plus++;
			if_retrieval[(i - j + N) & N] = false;
		}
		int plus_i = 0;
		for(int j = 1; j + K - 1 < N; j++){
			if(jewelry[(i + K + j - 1 + N) % N] == '0'){
				break;
			}
			blue_gem_plus++;
			plus_i++;
			if_retrieval[(i + j) & N] = false;
		}
		i += plus_i;
		double ans_tmp = (blue_gem_plus + blue_gem) / double(blue_gem_plus + K);
		if(ans < ans_tmp){
			ans = ans_tmp;
		}
	}
	cout << setprecision(10) << ans << endl;
}