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.HashMap;
import java.util.Scanner;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Queue;
public class Codeforsess {
 
    public static void main(String[] args) {

        Scanner scan = new Scanner(System.in);
        int n = scan.nextInt();
        String str = scan.next();
        String s1 ="";
        String s2 ="";
        int max = 0 ;
        for(int i = 0 ; i <str.length()-1;i++){
            int count = 0 ;
            s1="";
            for(int j =i+1;j+1!=str.length();j++ ){
                if(str.charAt(i)==str.charAt(j)&&str.charAt(i+1)==str.charAt(j+1));
                count++;
              
            }
            if(count>=max){
                max=count;
              
               s1=s1+(char)str.charAt(i)+s1+(char)str.charAt(i+1);
               s2=s1;   
            }
        }
        System.out.println(s2);
        
        
             
    }
}
    

    
 