#include<bits/stdc++.h>
#define int long long
using namespace std;
int n,k,l,r,cnt0=0,cnt1=0;
string s;
bool check(int num){
    cnt0=0,cnt1=0;
    int cnt=0;
    for (int i = 0; i < n; i++){
        if(s[i]=='1') cnt1++;
        else cnt0++;
        if((cnt1*cnt0)>num){
            cnt++;
            cnt1=cnt0=0;
            if(s[i]=='1') cnt1++;
            else cnt0++;
        }
    }
    //if(cnt0!=0 or cnt1!=0) cnt++;
    if(cnt>=k) return 1;
    return 0;
}
signed main(){
    ios::sync_with_stdio(false);
    cin.tie(0);cout.tie(0);
    cin>>n>>k;
    cin>>s;
    if(n<=k){
        cout<<0;
        return 0;
    }
    l=0;
    for (int i = 0; i < n; i++)
    {
        if(s[i]=='1') cnt1++;
        else cnt0++;
    }
    r=cnt0*cnt1;
    if(k==1){
    	cout<<r;
    	return 0;
	}
	r+=2;
    while(l<r){
        int mid=(l+r)>>1;
        if(check(mid)) l=mid+1;
        else r=mid-1;
    }
    cout<<l;
    return 0;
}

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