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 twoGram {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		int length = sc.nextInt();
		String palavra = sc.next();
		int maiorOcor = 0;
		String resposta = "";
		char[] palavraArray = palavra.toCharArray();
		for(int i = 0; i < length - 2; i++) {
			String substring = "" + palavraArray[i] + palavraArray[i+1];
			int ultIndex = 0;
			int count = 0;
			while(ultIndex != -1) {
				ultIndex = palavra.indexOf(substring, ultIndex);
				
				if(ultIndex != -1) {
					count++;
					ultIndex += substring.length();
				}
			}
			if(count > maiorOcor) {
				resposta = substring;
				maiorOcor = count;
			}
		}
		
		System.out.println(resposta);
	}
}
					 		 	 		  	 		 				 		 			