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 test2 {

    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        int n = input.nextInt();
        String s = input.next();
        input.close();

        HashMap<String, Integer> hashMap = new HashMap<>();
        char c1 = s.charAt(0);
        char c2;
        for(int i = 1; i<n; i++) {
            c2 = s.charAt(i);

            try {
                hashMap.put("" + c1 + c2, hashMap.get("AB") + 1);
            }
            catch (NullPointerException e){
                hashMap.put("" + c1 + c2, 1);
            }



            c1 = c2;
        }



        //get max value for eachEntry
        Map.Entry<String, Integer> maxEntry = null;

        for (Map.Entry<String, Integer> entry : hashMap.entrySet())
        {
            if (maxEntry == null || entry.getValue().compareTo(maxEntry.getValue()) > 0)
            {
                maxEntry = entry;
            }
        }

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


    }
}
