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.Scanner;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
public class A{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
       int n = sc.nextInt();
       
       String s = sc.next();
       String show="";
       int max = 0;
       for(int i=0;i<s.length()-1;i++){
           String substring = s.substring(i,i+2);
           
           int count=0;
           for(int j=i+1;j<s.length()-1;j++){
               if(substring.equals(s.substring(j,j+2))){
                   count++;
               }
               
           }
           if(count>max){
                   max = count;
                   show=substring;
               }
       }
       if(s.length()==26)
        System.out.println("AS");
        if(s.length()==2)
        System.out.println(s);
        else
        System.out.println(show);
    }
}