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.io.IOException;
import java.util.Scanner;
import java.util.*;

public class Main {

	public static void main(String[] args) throws IOException {
		// TODO Auto-generated method stub
		// CodeForces
		Scanner in = new Scanner(System.in);	
		int n = in.nextInt();
		String s = in.next();
		int res = 0;
		String ans="";
		for(int i=0; i<n-1; i++) {
			int cur=0;
			for(int j=0; j<n-1; j++) {
				
				if(s.charAt(j)==s.charAt(j) && s.charAt(j+1)==s.charAt(i+1)) {
					cur++;
				}
				if(res<cur) {
					res=cur;
					ans = ""+s.charAt(i)+s.charAt(i+1);
				}
			}
		}
		System.out.println(ans);
	}
}

