#include <iostream>
#include <string>
#include <cstdlib>

using namespace std;

int main() {
	int n, k;
	string s;
	cin >> n >> k >> s;
	int rewardInABox = 0;
	for (int i = 0; i < n; ++i) {
		rewardInABox += (int)(s[i] - '0');
	}
	int boxesToAnalyze = max(0, k - n * 2) / n;
	k -= n * boxesToAnalyze;
	int toBuyInABox = max(0, n - rewardInABox);
	int toBuy = toBuyInABox * boxesToAnalyze;
	int reward = 0;
	for (int i = 0; i < k; ++i) {
		if (reward > 0) {
			--reward;
		} else {
			++toBuy;
		}
		reward += (int)(s[i % n] - '0');
	}
	cout << toBuy << endl;
	return 0;
}