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.*;
import java.util.Scanner;

public class TwoGram {
	public static void main(String args[]) {
		int n;
		Scanner scan = new Scanner(System.in);

		n = scan.nextInt();

		String s = scan.next();

		int count = 0;
		String maxStr = null;
		int max = 0;

		for (int i=0; i<n-1; i++) {
			String sub = s.substring(i, i+2);

			for (int j=i+1; j<n-1; j++) {
				if (sub.equals(s.substring(j, j+2))) {
					count++;
				}
				if (count>max) {
					max = count;
					if (max != 0) {
					maxStr = sub;
				}
				}
			}
			count = 0;
		}

		System.out.println(maxStr);
	}
}