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;


public class Two_gram {
    public static void main(String[] args) {
       Scanner sc=new Scanner(System.in);
       int r=sc.nextInt();
       String str=sc.next();
       char x[]=str.toCharArray();
       int max=0,max1=0;
       char a,b,c = 0,d = 0,q = 0,w = 0;
       
        for (int i = 0; i < r-1; i++) {
            a=x[i];
            b=x[i+1];
            
             for (int j= 0; j < r-i-1; j++) {
            c=x[j];
            d=x[j+1];
                
            if(a==c&&d==b)
                max++;
            
        }
             if(max>max1)
            max1=max;
             q=c;
             w=d;
             
            
        }
 
        System.out.println(q+""+w);
        
       
    }
    
}
