#include "bits/stdc++.h"
using namespace std;
#define rep(i,n) for(int (i)=0;(i)<(int)(n);++(i))
#define rer(i,l,u) for(int (i)=(int)(l);(i)<=(int)(u);++(i))
#define reu(i,l,u) for(int (i)=(int)(l);(i)<(int)(u);++(i))
static const int INF = 0x3f3f3f3f; static const long long INFL = 0x3f3f3f3f3f3f3f3fLL;
typedef vector<int> vi; typedef pair<int, int> pii; typedef vector<pair<int, int> > vpii; typedef long long ll;
template<typename T, typename U> static void amin(T &x, U y) { if (y < x) x = y; }
template<typename T, typename U> static void amax(T &x, U y) { if (x < y) x = y; }

int main() {
	int N; int K;
	while (~scanf("%d%d", &N, &K)) {
		char buf[500001];
		scanf("%s", buf);
		string A = buf;
		A += buf;
		double lo = 0, up = 1;
		rep(kk, 35) {
			double mid = (lo + up) * .5;
			double curSumR = 0;
			double curSumL = 0, minSumL = 1e99;
			double maxSum = -1e99;
			int li = 0;
			rep(ri, N * 2) {
				for (; li + K <= ri; ++ li) {
					amin(minSumL, curSumL);
					curSumL += (A[li] - '0') - mid;
				}
				amax(maxSum, curSumR - minSumL);
				curSumR += (A[ri] - '0') - mid;
			}
			(maxSum >= 0 ? lo : up) = mid;
		}
		double ans = (lo + up) * .5;
		printf("%.10f\n", ans);
	}
	return 0;
}