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

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

    long long N,A; cin >> N >> A;
    string s; cin >> s;

    vector<long long> one(N);
    for(int i=0; i<N; i++){
        if(s.at(i) == '1') one.at(i) = 1;
        if(i) one.at(i) += one.at(i-1);
    }
    int rate = 1200,solve = 0;
    int idx = 0;
    for(auto c : s){
        idx++;
        if(c == '0') rate--;
        else if(rate < 1200) rate++,solve++;
        if(solve == A){cout << idx << endl; return 0;}
    }
    int memo = rate,solve2 = 0;
    vector<long long> second;
    for(auto c : s){
        if(c == '0') rate--;
        else if(rate < 1200) rate++,solve2++;
        second.push_back(solve2);
    }
    long long answer = N; A -= solve;
    long long loop = (A-1)/solve2; A--; A %= solve2,A++;
    answer += N*loop;
    for(int i=0; i<N; i++) if(second.at(i) == A){answer += i+1; break;}
    cout << answer << "\n";
}