#include <cstdio>
#include <iostream>
#include <string>
#include <sstream>
#include <stack>
#include <algorithm>
#include <cmath>
#include <queue>
#include <map>
#include <set>
#include <cstdlib>
#include <bitset>
#include <tuple>
#include <assert.h>
#include <deque>
#include <bitset>
#include <iomanip>
#include <limits>
#include <chrono>
#include <random>
#include <array>
#include <unordered_map>
#include <functional>
#include <complex>

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 long long MAX = 5100000;
const long long INF = 1LL << 60;
const long long mod = 1000000007LL;
//const long long mod = 998244353LL;

using namespace std;
typedef unsigned long long ull;
typedef long long ll;

ll N, K;
string s;
vector<ll> sum;

bool check(ll x) {
	ll cnt = x;
	ll re = x / N * sum[N] + sum[x % N];
	while (re > 0 && cnt < K) {
		ll start = cnt % N;
		cnt += re;
		re = re / N * sum[N] + sum[start + re%N] - sum[start];
	}
	if (cnt >= K) return true;
	else return false;
}
int main()
{
	/*
	cin.tie(nullptr);
	ios::sync_with_stdio(false);
	*/

	scanf("%lld %lld", &N, &K);
	cin >> s;
	sum.resize(2 * N + 1);
	for (ll i = 0; i < N; i++) {
		sum[i + 1] = sum[i] + (s[i] - '0');
	}
	for (ll i = N; i < N * 2; i++) {
		sum[i + 1] = sum[i] + (s[i - N] - '0');
	}
	ll l = 0;
	ll r = K + 5;
	while (r - l > 1) {
		ll mid = (l + r) >> 1;
		if (check(mid)) r = mid;
		else l = mid;
	}
	cout << r << endl;
	return 0;
}