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.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Scanner;

public class TwoGram
{
    public static void main(String [] args)
    {
        Scanner input = new Scanner(System.in);
        int n = input.nextInt();
        String str = input.next();

        List<String> list = new ArrayList<>();
        List<Integer> counters = new ArrayList<>();

        for(int i = 0; i < str.length() - 1; i++)
                list.add(str.substring(i, i + 2));

        int j = 0, counter = 0;
        while(j < list.size())
        {
            for(int i = 0; i < str.length() - 1; i++)
            {
                if(str.substring(i, i + 2).equals(list.get(j)))
                {
                    counter ++;
                    counters.add(counter);
                }
            }
            counter = 0;
            j ++;
        }

        int max = Collections.max(counters);
        int maxIndex = counters.indexOf(max);

        System.out.println(list.get(maxIndex));
    }
}