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.*;
import java.math.*;

public class CP {
	public static void main(String[] args) {
		Scanner sc = new Scanner (System.in);
		int n = sc.nextInt();
		String s = sc.next();
		sc.close();
		String temp = "";
		int counter = 0;
		int max = Integer.MIN_VALUE;
		for(int i = 0; i < s.length() - 1; i++) {
			String a = s.substring(i, i + 2);
			counter = 0;
			for(int j = i + 1; j < s.length() - 1; j++) {
				if(s.substring(j, j + 2).equals(a)) {
					counter++;
					if(counter > max) {
						max = counter;
						temp = a;
					}
				}
			}
		}
		System.out.println(temp);
	}
}
