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.Scanner;

public class Main {
    public static void main(String arg[]){
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        sc.nextLine();
        String str = sc.nextLine();
        sc.close();
        int count = 0,maxCount = -1;
        String ans = "";
        for(int i = 0; i < str.length() - 2; i++){
            String cut = str.substring(i, i +2);
            count = 0;
            for (int j = 0; j < str.length() - 2; j++) {
                if(str.substring(j,j+2).equals(cut))count++;
            }
            if(count > maxCount){
                maxCount = count;
                ans = cut;
            }
        }
        System.out.println(ans);
    }
}