#include <bits/stdc++.h>
//#define int long long
//#define USE_FREOPEN
//#define MUL_TEST
#define FILENAME "sapphire"
using namespace std;
//constexpr double eps = 1e-6;
int n,k;
string s;

bool chk(double x) {
	vector<double> sa(2 * n + 1,0.0);
	for (int i = 0; i < 2 * n; i++) {
		int val = (s[i] == '1') ? 1 : 0;
		sa[i + 1] = sa[i] + (val - x);
	}
	deque<int> q;
	for (int i = 1; i <= 2 * n; i++) {
		int imk = i - k,imn = max(i - n,0);
		if (imk >= 0) {
			while (q.size() && sa[q.back()] >= sa[imk]) q.pop_back();
			q.push_back(imk);
		}
		while (q.size() && q.front() < imn) q.pop_front();
		if (i >= k) {
			if (q.size()) {
				if (sa[i] - sa[q.front()] >= -1e-9) return 1;
			}
		}
	}
	return 0;
}

void solve() {
	cin >> n >> k >> s;
	s = s + s;
	
	double l = 0.0,r = 1.0;
	for (int i = 0; i < 100; i++) {
		double mid = (l + r) / 2.0;
		if (chk(mid)) l = mid;
		else r = mid;
	}
	printf("%.15lf\n",r);
}

signed main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr); cout.tie(nullptr);

	#ifdef USE_FREOPEN
		freopen(FILENAME ".in","r",stdin);
		freopen(FILENAME ".out","w",stdout);
	#endif
	int _ = 1;
	#ifdef MUL_TEST
		cin >> _;
	#endif
	while (_--)
		solve();
	_^=_;
	return 0^_^0;
}