/**
	author:  shobonvip
	created: 2025.09.07 09:11:53
**/

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;

struct S {
	ll sum, mx;
};

S op(S a, S b) {
	return S{a.sum + b.sum, max(a.mx, a.sum + b.mx)};
}

S e() {
	return S{0, 0};
}

int main(){
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);

	ll n, a; cin >> n >> a;
	string s; cin >> s;
	vector<S> rui(n + 1);
	vector<ll> rui_1num(n + 1);
	rui[0] = e();
	for (int i=0; i<n; i++) {
		if (s[i] == '0') {
			rui[i+1] = op(rui[i], S{-1, 0});
		} else {
			rui[i+1] = op(rui[i], S{1, 1});
			rui_1num[i+1] += 1;
		}
		rui_1num[i+1] += rui_1num[i];
	}

	auto check = [&](ll t) -> ll {
		ll p = t / n;
		ll q = t % n;
		S v = e();
		ll nums = rui_1num[n] * p + rui_1num[q];
		{
			S w = rui[n];
			while (p > 0) {
				if (p & 1) v = op(v, w);
				w = op(w, w);
				p >>= 1;
			}
		}
		v = op(v, rui[q]);
		return nums - v.mx;
	};

	ll ub = 2e18; // OK
	ll lb = 0; // NG
	while (ub - lb > 1) {
		ll t = (ub + lb) / 2;
		if (check(t) >= a) {
			ub = t;
		}else {
			lb = t;
		}
	}
	cout << ub << endl;
}