TamimHolmes

#include <iostream>
#include<vector>
#define fastio  ios_base::sync_with_stdio(0);cin.tie(NULL);

using namespace std;
vector<int> prefix_function(string s) {
    int n = (int)s.length();
    vector<int> pi(n);
    for (int i = 1; i < n; i++) {
        int j = pi[i-1];
        while (j > 0 && s[i] != s[j])
            j = pi[j-1];
        if (s[i] == s[j])
            j++;
        pi[i] = j;
    }
    return pi;
}

int KMPSearch(string pat, string txt, int i){
    int lenp=(int)pat.length();
    int lent=(int)txt.length();
    vector<int>lps=prefix_function(pat);

    int j=0;
    int total=0;
    while(i<lent){
        if(pat[j]==txt[i]){
            j++;
            i++;
        }

    if(j==lenp){
        total++;
        j=lps[j-1];
    }
    else if(i<lent && pat[j]!=txt[i]){
        if(j!=0){
            j=lps[j-1];
        }
        else i++;
    }
}
return total;
}

int main(){
  int L,len,total,highest;
  string x,tmp,temp;
  while(cin>>L,cin>>x){
    highest=-999;
    len=x.length();
    len=len-L;
    for(int i=0;i<len;i++){
      tmp=x.substr(i,L);
      if(temp==tmp)continue;
      total=KMPSearch(tmp,x,i);
      if(total>highest){
        highest=total;
        temp=tmp;
      }
    }
    cout<<temp<<'\n';
  }
}