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

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    ll n, a;
    string s;
    cin >> n >> a >> s;
    vector<ll> ac(21), rt(21);
    ll rtg = 0, cor = 0;
    for(int j = 0; j < 20; j++){
        for(int i = 0; i < n; i++){
            if(s[i] == '1'){
                if(rtg < 0) cor++, rtg++;
                if(cor == a){
                    cout << j * n + i + 1 << '\n';
                    return 0;
                }
            }else{
                rtg--;
            }
        }
        ac[j + 1] = cor;
        rt[j + 1] = rtg;
    }
    assert(ac[20] - ac[19] == ac[19] - ac[18]);
    assert(rt[20] - rt[19] == rt[19] - rt[18]);
    ll d = (a - ac[20] - 1) / (ac[20] - ac[19]);
    rtg += d * (rt[20] - rt[19]);
    cor += d * (ac[20] - ac[19]);
    d += 20;
    for(int i = 0; i < s.size(); i++){
        if(s[i] == '1'){
            if(rtg < 0) cor++, rtg++;
            if(cor == a){
                cout << d * n + i + 1 << '\n';
                return 0;
            }
        }else{
            rtg--;
        }
    }
    cerr << "enter" << '\n';
}