#include <stdio.h>
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using mint = modint1000000007;
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf 100000000000000000


int main(){
	
	int N;
	long long K;
	cin>>N>>K;
	
	string S;
	cin>>S;
	
	long long cur = 0LL,mini = 0LL;
	
	rep(i,N){
		cur--;
		mini = min(mini,cur);
		cur += S[i] - '0';
		if(K-1==i){
			cout<<-mini<<endl;
			return 0;
		}
	}
	
	if(cur>=0){
		cout<<-mini<<endl;
		return 0;
	}
	
	long long ans = abs((K/N - 1) * cur + mini);
	cur *= K/N;
	
	rep(i,K%N){
		cur--;
		mini = min(mini,cur);
		cur += S[i] - '0';
	}
	ans = max(ans,-mini);
	cout<<ans<<endl;
	
	return 0;
}