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 Codeforces {

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

        scanner.nextLine();
        String line = scanner.nextLine();

        Set<String> set = new HashSet<>();
        String res = "";
        int max = 0;

        for (int i = 0; i < line.length() - 1; i++) {
            String sub = line.substring(i, i + 2);
            if (!set.add(sub)) continue;
            int tmp = count(line, sub);
            if (tmp > max) {
                max = tmp;
                res = sub;
            }
        }

        System.out.println(res);
    }

    private static int count(String line, String sub) {
        if (sub.charAt(0) != sub.charAt(1))
            return (line.length() - line.replace(sub, "").length()) / 2;
        int res = 0;
        while (line.contains(sub)) {
            line = line.replaceFirst(String.valueOf(sub.charAt(0)), "");
            res++;
        }
        return res;
    }

}
