#include "bits/stdc++.h"
using namespace std;

int main(){
	int N, K;
	cin >> N >> K;
	string S;
	cin >> S;
	int hit = 0;
	for (int i = 0; i < S.size(); i++)
	{
		hit += S[i] - '0';
	}

	if (hit >= N){
		int nokori = 0;
		int ans = 0;
		for (int i = 0; i < S.size() && i < K; i++)
		{
			nokori -= 1;
			while (nokori < 0) {
				ans++;
				nokori++;
			}
			nokori += S[i] - '0';
		}
		cout << ans << endl;
	}
	else{
		long long ans = K;
		ans -= K / S.size() * hit;
		for (int i = 0; i < K % S.size(); i++)
		{
			ans -= S[i] - '0';
		}

		int waste = 0;
		int temp = 0;
		int change = 0;
		for (int i = K - 1; i >= 0 && change < 100; i--)
		{
			temp += S[i % S.size()] - '0';
			if (temp > waste){
				waste = temp;
				change = 0;
			}
			temp--;
			change++;
		}
		ans += waste;
		cout << ans << endl;
	}
	
}