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.Map;
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        long num = sc.nextLong();
        Map<String, Integer> map = new HashMap<>();
        char[] str = sc.next().toCharArray();
        for (int i = 1; i < str.length; i++) {
            String s = String.valueOf(str[i - 1]) + String.valueOf(str[i]);
            if (map.containsKey(s))
                map.put(s, map.get(s) + 1);
            else
                map.put(s, 1);
        }
        Map.Entry<String, Integer> maxEntry = (Map.Entry<String, Integer>) map.entrySet().toArray()[0];
        for (Map.Entry<String, Integer> entry : map.entrySet()) {
            if (maxEntry.getValue() < entry.getValue())
                maxEntry = entry;

        }
        System.out.println(maxEntry.getKey());
    }

}
