#include <bits/stdc++.h>
using namespace std;
#define GET_MACRO(a, b, c, NAME, ...) NAME
#define rep(...) GET_MACRO(__VA_ARGS__, rep3, rep2)(__VA_ARGS__)
#define rep2(i, a) rep3 (i, 0, a)
#define rep3(i, a, b) for (int i = (a); i < (b); i++)
#define repr(...) GET_MACRO(__VA_ARGS__, repr3, repr2)(__VA_ARGS__)
#define repr2(i, a) repr3 (i, 0, a)
#define repr3(i, a, b) for (int i = (b) - 1; i >= (a); i--)
template<class T1, class T2> inline bool chmin(T1 &a, T2 b) { return b < a && (a = b, true); }
template<class T1, class T2> inline bool chmax(T1 &a, T2 b) { return a < b && (a = b, true); }
using ll = long long;

int main() {
	int n, k;
	string s;
	cin >> n >> k >> s;

	ll buy = 0, num = 0, ans = 0;
	if (k >= n) {
		rep (i, n) {
			if (num == 0) buy++;
			else num--;
			num += s[i] - '0';
		}
		if (num != 0) {
			ll x = (buy + num - 1) / num;
			ans = x * (buy + buy - (x - 1) * num) / 2;
			num *= x;
		} else {
			ans = buy * k / n;
		}
		k %= n;
	}
	rep (i, k) {
		if (num == 0) ans++;
		else num--;
		num += s[i] - '0';
	}
	cout << ans << endl;
	return 0;
}