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.*;
public class asdas
{
         public static void main(String args[])throws Exception
         {
                  Scanner s=new Scanner(System.in);
                  int n=s.nextInt();
                  String str=s.nextLine();
                  int max=Integer.MIN_VALUE;char ca=' ';char cb=' ';
                  for(int i=0;i<str.length()-2;i++)
                  {
                           char ch1=str.charAt(i);int count=0;
                           char ch2=str.charAt(i+1);
                           for(int j=i+1;j<str.length()-1;j++)
                           {
                               char ch3=str.charAt(j);
                               char ch4=str.charAt(j+1);
                               if(ch1==ch3&&ch2==ch4)
                               count++;
                           }
                           if(count>max)
                           {
                           max=count;ca=ch1;cb=ch2;}
                  }
                  System.out.println(ca+""+cb);
                  
         }
}