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.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.math.BigInteger;
import java.util.*;

import static java.lang.Math.max;




public class Main {

 

    public static void main(String args[]) throws Exception {
        Scanner sc = new Scanner(System.in);

        int n = sc.nextInt();

        String word = sc.next();

        int max = 0;
        String result = "";
        for (int i = 0; i < n - 1; ++i) {
            int count = 1;
            String gram = word.charAt(i) + "" + word.charAt(i + 1);
            String occ = gram;
            for (int j = i + 1; j < n-1; ++j) {
                if ((word.charAt(j) + "" + word.charAt(j + 1)).equals(gram)) {
                    count++;
                    occ = gram;
                }

            }
            if (count > max) {
                max = count;
                result = occ;
            }
        }

        System.out.println(result);
    }
}