#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for (int i = 0; i< (n); ++i)
#define repi(i, a, b) for (int i = (a); i < (b); ++i)
#define all(x) (x).begin(), (x).end()
#define fore(i, a) for(auto &i:a)
using ll = long long;
using int64 = long long;
#define DEBUG(x) cerr << #x << ": "; for (auto _ : x) cerr << _ << " "; cerr << endl;
const int64 infll = (1LL << 62) - 1;
const int inf = (1 << 30) - 1;

struct IoSetup {
	IoSetup() {
		cin.tie(nullptr);
		ios::sync_with_stdio(false);
		cout << fixed << setprecision(10);
		cerr << fixed << setprecision(10);
	}
} iosetup;

template <typename T1, typename T2>
ostream& operator<<(ostream& os, const pair<T1, T2>& p) {
	os << p.first << " " << p.second;
	return os;
}

template <typename T1, typename T2>
istream& operator>>(istream& is, pair<T1, T2>& p) {
	is >> p.first >> p.second;
	return is;
}

template <typename T>
ostream& operator<<(ostream& os, const vector<T>& v) {
	for (int i = 0; i < (int)v.size(); i++) {
		os << v[i] << (i + 1 != v.size() ? " " : "");
	}
	return os;
}

template <typename T>
istream& operator>>(istream& is, vector<T>& v) {
	for (T& in : v) is >> in;
	return is;
}

template <typename T1, typename T2>
inline bool chmax(T1& a, T2 b) {
	return a < b && (a = b, true);
}

template <typename T1, typename T2>
inline bool chmin(T1& a, T2 b) {
	return a > b && (a = b, true);
}

template <typename T = int64>
vector<T> make_v(size_t a) {
	return vector<T>(a);
}

template <typename T, typename... Ts>
auto make_v(size_t a, Ts... ts) {
	return vector<decltype(make_v<T>(ts...))>(a, make_v<T>(ts...));
}

template <typename T, typename V>
typename enable_if<is_class<T>::value == 0>::type fill_v(T& t, const V& v) {
	t = v;
}

template <typename T, typename V>
typename enable_if<is_class<T>::value != 0>::type fill_v(T& t, const V& v) {
	for (auto& e : t) fill_v(e, v);
}

template <typename F>
struct FixPoint : F {
	explicit FixPoint(F&& f) : F(std::forward<F>(f)) {}

	template <typename... Args>
		decltype(auto) operator()(Args&&... args) const {
			return F::operator()(*this, std::forward<Args>(args)...);
		}
};

template <typename F>
inline decltype(auto) MFP(F&& f) {
	return FixPoint<F>{std::forward<F>(f)};
}
int main(){
	ll n, a;
	cin >> n >> a;
	string s;
	cin >> s;
	vector<ll> idx1, idx2;
	ll rate = 1200;
	rep(i, n){
		if(s[i] == '0')rate--;
		else{
			if(rate < 1200){
				rate++;
				idx1.push_back(i);
			}
		}
	}
	rep(i, n){
		if(s[i] == '0')rate--;
		else{
			if(rate < 1200){
				rate++;
				idx2.push_back(i);
			}
		}
	}
	DEBUG(idx1);
	DEBUG(idx2);
	ll fi = idx1.size(), se = idx2.size();
	ll ans;
	if(a <= fi){
		ans = idx1[a-1]+1;
	}
	else{
		a -= fi;
		auto[d, r] = div(a, se);
		if(r == 0){
			ans = d*n+idx2[se-1]+1;
		}
		else{
			ans = (d+1)*n+idx2[r-1]+1;
		}
	}
	cout << ans << "\n";


}