#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
istream &operator>>(istream &is, modint &a) { long long v; is >> v; a = v; return is; }
ostream &operator<<(ostream &os, const modint &a) { return os << a.val(); }
istream &operator>>(istream &is, modint998244353 &a) { long long v; is >> v; a = v; return is; }
ostream &operator<<(ostream &os, const modint998244353 &a) { return os << a.val(); }
istream &operator>>(istream &is, modint1000000007 &a) { long long v; is >> v; a = v; return is; }
ostream &operator<<(ostream &os, const modint1000000007 &a) { return os << a.val(); } 

typedef long long ll;
typedef vector<vector<int>> Graph;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
#define FOR(i,l,r) for (int i = l;i < (int)(r); i++)
#define rep(i,n) for (int i = 0;i < (int)(n); i++)
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define my_sort(x) sort(x.begin(), x.end())
#define my_max(x) *max_element(all(x))
#define my_min(x) *min_element(all(x))
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
const int INF = (1<<30) - 1;
const ll LINF = (1LL<<62) - 1;
const int MOD = 998244353;
const int MOD2 = 1e9+7;
const double PI = acos(-1);
vector<int> di = {1,0,-1,0};
vector<int> dj = {0,1,0,-1};

#ifdef LOCAL
#  include <debug_print.hpp>
#  define debug(...) debug_print::multi_print(#__VA_ARGS__, __VA_ARGS__)
#else
#  define debug(...) (static_cast<void>(0))
#endif

int main(){
    cin.tie(0);
    ios_base::sync_with_stdio(false);
    const int L = 1200;

    ll N, A; 
    cin >> N >> A;
    string S;
    cin >> S;

    int X0 = L;
    rep(i, N){
        if(S[i] == '0') X0--;
        else if(X0 < L) {
            X0++;
            if(--A == 0) {
                cout << i + 1 << endl;
                return 0;
            }
        }
    }

    int X1 = X0;
    vector<int> cnt(N + 1);
    rep(i, N){
        cnt[i + 1] = cnt[i];
        if(S[i] == '0') X1--;
        else if(X1 < L) {
            X1++;
            cnt[i + 1]++;
            if(--A == 0) {
                cout << i + 1 + N << endl;
                return 0;
            }
        }
    }

    ll M = cnt[N];
    ll ans = 2LL * N + (A / M - (A % M == 0)) * N;
    A -= (A / M - (A % M == 0)) * M;

    rep(i, N) {
        if(A <= cnt[i + 1]){
            cout << ans + i + 1 << endl;
            return 0;
        }
    }
}