#ifndef ONLINE_JUDGE
#define _GLIBCXX_DEBUG //[]で配列外参照をするとエラーにしてくれる。上下のやつがないとTLEになるので注意 ABC311Eのサンプル4みたいなデバック中のTLEは防げないので注意
#endif
#include <bits/stdc++.h>
#include <algorithm>
#include <cmath> // M_PIを使用するため
#include <atcoder/all>
using namespace std;
using namespace atcoder;

using ll = long long;
using ld = long double;
#define rep(i, n) for (ll i = 0; i < (ll)(n); i++)
#define rrep(i, n) for (ll i = (ll)n - 1; i >= 0; --i)

const ll INF = (1LL << 62);
const ll null = -1LL;
template <typename T>
using vc = vector<T>; // prioriy_queueに必要なのでここにこれ書いてます
template <typename T>
using vv = vc<vc<T>>;
template <typename T>
using vvv = vv<vc<T>>;
using vl = vc<ll>;
using vvl = vv<ll>;
using vvvl = vv<vl>;
using vvvvl = vv<vvl>;
using vs = vc<string>;
using vvs = vv<string>;
using vb = vc<bool>;
using vvb = vc<vb>;
using P = pair<ll, ll>;

template <class T>
istream &operator>>(istream &i, vc<T> &v)
{
    rep(j, size(v)) i >> v[j];
    return i;
}
// それぞれ「下，上，右，左」に対応
int dx[4] = {1, -1, 0, 0};
int dy[4] = {0, 0, 1, -1};

#define nall(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()

template <class T>
bool chmin(T &a, T b)
{
    if (a > b)
    {
        a = b;
        return true;
    }
    return false;
}
template <class T>
bool chmax(T &a, T b)
{
    if (a < b)
    {
        a = b;
        return true;
    }
    return false;
}

#define pb push_back
#define eb emplace_back
#define em emplace
#define pob pop_back
#define next_p(v) next_permutation(v.begin(), v.end())

void solve()
{
    ll n, a; cin >> n >> a;
    string s; cin >> s;
    vl cnt(2,0);
    ll start = 1200;
    ll sum = 0;
    rep(i,n) {
        cnt[(ll)(s[i] - '0')]++;
        if (s[i] == '1') {
            if (start < 1200ll) ++sum; 
            start = min(start + 1ll, 1200ll);
        }
        else --start;
        if (sum == a) {
            cout << i + 1ll << endl;
            return;
        }
    }

    ll num = (a - sum - 1ll) / min(cnt[0],cnt[1]);
    ll ans = (num + 1ll) * n, rest = a - sum - min(cnt[0],cnt[1]) * num;
    if (cnt[0] < cnt[1]) {
        for (char c : s) {
            ++ans;
            if (c == '1') {
                if (start < 1200ll) --rest;
                start = min(start + 1ll, 1200ll);
            } else --start;
            if (rest <= 0) {
                break;
            }
        }
    } else {
        for (char c : s) {
            ++ans;
            if (c == '1') --rest;
            if (rest <= 0) {
                break;
            }
        }
    }

    cout << ans << endl;

}

int main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    ll testcases = 1ll;
    // cin >> testcases;
    rep(_, testcases) solve();

    return 0;
}