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 ProbB
{
    public static void main(String[] args)
    {
        Scanner input = new Scanner(System.in);

        int length = input.nextInt();

        String s = input.next();

        String mostCommon = "";

        int mostTimes = 0;

        for(int i = 0; i < length; i++)
        {
           if(i+2 <= length)
           {
               String gram = s.substring(i, i + 2);

               int gCount = 0;

               for(int j = 0; j < length; j++)
               {
                   //check in bounds and cound
                   if(j+2 <= length && s.substring(j, j+2).equals(gram))
                   {
                       gCount++;
                   }
               }

               if(gCount > mostTimes)
               {
                   mostCommon = gram;
                   mostTimes = gCount;
               }
           }
        }

        System.out.println(mostCommon);
    }
}
