#include <iostream>
#include<bits/stdc++.h>

using namespace std;

int main() {
	// your code goes here
	int n,k;
	string a;
	cin>>n>>k;
	cin>>a;
	vector<int> prefix(n+1,0);
	vector<int> suffix(n+1,0);
	for(int i=1;i<=n;i++){
		if(a[i-1]=='1'){
			prefix[i]=prefix[i-1]+1;
		}
		else{
			prefix[i]=0;
		}
	}
	for(int i=n-1;i>=0;i--){
		if(a[i]=='1'){
			suffix[i]=suffix[i+1]+1;
		}
		else{
			suffix[i]=0;
		}
	}
	
    int i=0;
    int j=k;
    int ans=0;
    
    
    while(i<n && j<n){
    	int consecutive = prefix[i]+suffix[j]+k;
    	ans = max(ans,consecutive);
    	i++;
    	j++;
    }
    cout<<ans<<endl;
    
	return 0;
}