#include <bits/stdc++.h>
//#include <atcoder/all>
using namespace std;
//using namespace atcoder;
//using mint = modint1000000007;
//const int mod = 1000000007;
//using mint = modint998244353;
//const int mod = 998244353;
//const int INF = 1e9;
//const long long LINF = 1e18;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep2(i,l,r)for(int i=(l);i<(r);++i)
#define rrep(i, n) for (int i = (n) - 1; i >= 0; --i)
#define rrep2(i,l,r)for(int i=(r) - 1;i>=(l);--i)
#define all(x) (x).begin(),(x).end()
#define allR(x) (x).rbegin(),(x).rend()
#define P pair<int,int>
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }

int main() {
	std::ios::sync_with_stdio(false);
	std::cin.tie(nullptr);
	int n; cin >> n;
	long long a; cin >> a;
	string s; cin >> s;
	// 1syuume
	int rate = 1200;
	int c0 = 0, c1 = 0;
	rep(i, s.size()) {
		if (s[i] == '0') {
			c0++;
			rate--;
		}
		else {
			c1++;
			if (rate < 1200) {
				rate++;
				a--;
				if (a == 0) {
					cout << i + 1 << endl;
					return 0;
				}
			}
		}
	}
	if (c0 >= c1) {
		long long ans = s.size();
		long long x = max(0LL, a / c1 - 1);
		ans += x * s.size();
		a -= x * c1;
		rep(i, n * 2) {
			if (s[i%n] == '1') {
				a--;
				if (a == 0) {
					ans += i + 1;
					break;
				}
			}
		}
		cout << ans << endl;
	}
	else {
		long long ans = s.size();
		long long x = max(0LL, a / c0 - 1);
		ans += x * s.size();
		a -= x * c0;
		rep(i, n * 2) {
			if (s[i%n] == '1') {
				if (rate < 1200) {
					rate++;
					a--;
					if (a == 0) {
						ans += i + 1;
						break;
					}
				}
			}
			else {
				rate--;
			}
		}
		cout << ans << endl;
	}

	return 0;
}