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 Main {


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

        int n = in.nextInt();
        String s = in.next();
        char ss = 0;
        char ee = 0;
        int res = 0;
        for(int i=0; i<n-1; i++) {
            int cnt = 0;
            for(int j=0; j<n-1; j++) {
                if(s.charAt(i)==s.charAt(j) && s.charAt(j+1)==s.charAt(i+1)) {
                    cnt++;
                }

            }
            if(res<cnt) {
                res = cnt;
                ss = s.charAt(i);
                ee = s.charAt(i+1);
            }
        }
        System.out.println(ss+""+ee);
}}
