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 sc = new Scanner(System.in);

        int length = sc.nextInt();
        sc.nextLine();
        String word = sc.nextLine();
        HashMap<String, Integer> occurrences = new HashMap<>();

        String firstPair = word.charAt(0) + "";
        firstPair += word.charAt(1);
        occurrences.put(firstPair, 1);
        AbstractMap.SimpleEntry<String, Integer> max = new AbstractMap.SimpleEntry<>(firstPair, 1);
        for (int i = 1; i < length - 1; i++) {
            String ordered = word.charAt(i) + "";
            ordered += word.charAt(i + 1);
            if (!occurrences.containsKey(ordered)) {
                occurrences.put(ordered, 1);
                continue;
            }
            int found = occurrences.get(ordered) + 1;
            occurrences.put(ordered, found);
            if(found > max.getValue())
                max = new AbstractMap.SimpleEntry<>(ordered, found);

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