15
MIRZOYANOVECLOX
 23
AABBBAAACCCCCAAADDDDDDD
 2
QA
 11
GGRRAATTZZZ
 6
AZAZAZ
 10
SQSQSQSQTG
 3
KEK
 100
URXCAIZFIBNJTPCZHBQIBCILLPXZCFGMKKZMNPLCYGAVJVIBMCZEBSJWPSCPQDYCTTKPOKIJRSKIZPDGCHVOUTMPNECYORSFZFNC
 3
LOL
 2
WW
 8
PUTINVOR
 26
QWERTYUIOPASDFGHJKLZXCVBNM
 5
AZAZA
 9
EGORLETOV
 7
KADUROV
 9
MIKEPIDOR
 9
AAAAAAAAA
 100
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
 5
FUFEL
 50
NYQAHBYYOXLTRYQDMVENEMAQNBAKGLGQOLXNAIFNQTOCLNNQIA

MI
 DD
 QA
 ZZ
 AZ
 SQ
 KE
 IB
 LO
 WW
 PU
 QW
 AZ
 EG
 KA
 MI
 AA
 AA
 FU
 YQ


import java.util.*;
public class Twogram{
    public static void main(String[] args){
        Scanner sc=new Scanner(System.in);
        
        
       
           int n = sc.nextInt();
            
           String s = sc.next();
           
          int res =0;
          int count =0;
          String ans="";
          
          for(int i=0;i<n-1;i++){
              count =0;
              for(int j=0;j<n-1;j++){
                  if(s.charAt(i)==s.charAt(j) && s.charAt(i+1)==s.charAt(j+1)){
                      count++;
                      if(count>res){
                          res=count;
                          ans = s.substring(0,i) + s.substring(0,i+2);
                      }
                  }
              }
          }
           
           System.out.println(ans);
        
    }
}
