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.HashMap;
import java.util.Scanner;

public class Solution {
    public static void main(String[] args) {
        new Solution().solve();
    }

    public void solve() {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        String s = in.next();
        HashMap<String, Integer> map = new HashMap<>();
        int max = 0, count = 0;
        String res = "";
        for (int i = 0; i < n - 1; i++) {
            String subs = s.substring(i, i + 2);
            if (map.containsKey(subs)) {
                count = map.get(subs) + 1;
                map.put(subs, count);
            } else {
                map.put(subs, 1);
                count = 1;
            }
            if (count > max) {
                res = subs;
                max = count;
            }
        }
        System.out.println(res);

    }
}