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.io.*;
import java.lang.*;
public class vvv
{
    public static void main(String args[])
    {
        Scanner sc=new Scanner (System.in);
        int n=sc.nextInt();
        String w=sc.nextLine();
        String s=sc.nextLine();
        String sa[]=new String[n-1];
        int ar[]=new int[n-1];int p;
        for(int i=0;i<n-1;i++)
        {
            String s1=s.substring(i,i+2);
            sa[i]=s1;p=0;
            for(int j=0;j<n-1;j++)
            {
                String s2=s.substring(j,j+2);
                if(s1.equals(s2))
                p++;
            }
            ar[i]=p;
        }int m=0;
        for(int i=1;i<n-2;i++)
        {
            if(ar[i]>m)
            m=i;
        }
        System.out.println(sa[m]);
    }

        }