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 Main {

	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);

		Integer n = scanner.nextInt();
		scanner.nextLine();
		int[][] matrix = new int[26][26];
		String s = scanner.nextLine();
		s = s.toLowerCase();
		for (int i = 0; i < s.length() -1; i++) {
			char c1 = s.charAt(i);
			char c2 = s.charAt(i+1);
			matrix[c1-97][c2-97] ++;
		}
		int maximum = -1;
		int targetLine = -1;
		int targetColumn = -1;
		for(int i = 0; i < 26; i++) {
			for(int j = 0; j < 26; j++) {
				if(matrix[i][j] > maximum) {
					maximum = matrix[i][j];
					targetLine = i;
					targetColumn = j;
				}
			}
		}
		System.out.println(new String(new char[] {(char)(97+targetLine), (char)(97+targetColumn)}));
		scanner.close();
	}
}
