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

signed main(){
  int n,m; cin >> n >> m; string s; cin >> s;
  int x = 1200;
  for (int i(0);i < n;++i){
    if (s[i]=='0') --x;
    if (s[i]=='1'&&x<1200) ++x,--m;
    if (m==0) (cout << i+1 << endl),exit(0);
  }
  int y = x,z = 0;
  for (int i(0);i < n;++i){
    if (s[i]=='0') --y;
    if (s[i]=='1'&&y<1200) ++y,--m,++z;
    if (m==0) (cout << n+i+1 << endl),exit(0);
  }
  int r = 2*n+(m-1)/z*n; m -= (m-1)/z*z;
  for (int i(0);i < n;++i){
    if (s[i]=='0') --y;
    if (s[i]=='1'&&y<1200) ++y,--m,++z;
    if (m==0) (cout << r+i+1 << endl),exit(0);
  }
}