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.Arrays;
import java.util.Scanner;
import java.util.Set;
import java.util.TreeSet;

public class Main3 {

    public static void main(String[] args) {
        Scanner s = new Scanner(System.in);
        Set<String> numT = new TreeSet<>();
        int n = s.nextInt();
        s.nextLine();
        String word = s.nextLine();
        
        String[] lista = new String[word.length() - 1];
        for (int i = 0; i < word.length() - 1; i++) {
            lista[i] = word.substring(i, i + 2);
        }
        
        int [] times = new int[n];

        for (int i = 0; i < n-1; i++) {
            if (numT.contains(lista[i])) {
                ++times[i];
            } else {
                numT.add(lista[i]);
            }
        }
        
        Arrays.sort(times);
        System.out.println(lista[times[n-1]]);
    }

}
