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.Arrays;
import java.util.Scanner;

public class JavaApplication168 {

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

        int n = sc.nextInt();
        String arr[] = new String[60000];
        Arrays.fill(arr, "");
        String s = sc.next();
        String xx = "";
        for(int i=0; i<n-1; i++){
            arr[s.charAt(i)+s.charAt(i+1)%arr.length] += s.charAt(i)+""+s.charAt(i+1);
            if( arr[s.charAt(i)+s.charAt(i+1)%arr.length].length() > xx.length()){
                xx = arr[s.charAt(i)+s.charAt(i+1)%arr.length];
            }
        }
        System.out.println(xx.substring(0,2));
        
        
        
        
    }
}
