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 x= new Scanner(System.in);
        int a= x.nextInt();
        x.nextLine();
        String s=x.nextLine();
        int n=s.length();
        int count[]= new int[n-1];
        String c[]= new String[n-1];
        int i, j=0, k, max=0, z=0;
        for(i=0; i<n-1; i++)
        {
            for(k=i+1; k<n-1; k++)
            {
                if(s.substring(i,i+2).compareTo(s.substring(k, k+2))==0)
                {
                    count[j]++;
                    c[j]= s.substring(i, i+2);
                    j++;
                }
                
            }
        }
        max= count[0];
        for(i=0; i<j; i++)
        {
            if(count[i]>max)
            {
            max=count[i];
            z=i;
            }   
        }
        System.out.println(c[z]);
    }
}
        