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.*;

public class B {
	
	public static void main(String[] args) {
		int n;
		String str = "";
		Scanner scan = new Scanner(System.in);
		n = Integer.parseInt(scan.nextLine());
		str = scan.nextLine();
		int i = 0,j = 1;
		boolean flag = false;
		for(int k = 0;k < n-2;k++) {
			for(int m = i+1,o = j+1;m < n-1;m++,o++) {
				if((String.valueOf(str.charAt(i)) + String.valueOf(str.charAt(j))).equals(String.valueOf(str.charAt(m)) + String.valueOf(str.charAt(o)))){
					System.out.println(String.valueOf(str.charAt(i)) + String.valueOf(str.charAt(j)));
					flag = true;
					break;
				}
			}
			if(flag)
				break;
			i++;
			j++;
		}
	}
	
}