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

//package Codeforces479;

import java.util.Scanner;

public class BProblem {

	public static void main(String[] args) {
	
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		String str = sc.next(), res = "";
		int ans = 0, c = 0;
		for (int i = 0; i < str.length()-1; i++) {
			c = 0;
			for (int j = 0; j < str.length()-1; j++)
				if (str.substring(i,i+2).equals(str.substring(j, j+2))) c++;
			if (c > ans) {
				ans = c;
				res = str.substring(i, i+2);
			}
		}
		
		System.out.println(res);
		
	}
}
