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 median
{
	public static void main(String args[])
	{
		Scanner s = new Scanner(System.in);
		int n = s.nextInt();
		String word = s.next();
		char[] word_array = word.toCharArray();

		char current = word_array[0];
		char next = word_array[1];

		int[][] frequency = new int[26][26];
		int max = 0;
		int current_frequency = 0;
		String answer = "";

		for(int i = 0; i < n - 2; i++)
		{
			current_frequency = ++frequency[(int)current - 65][(int)next - 65];
			if(current_frequency > max)
			{
				max = current_frequency;
				answer = Character.toString(current) + Character.toString(next);
			}
			current = next;
			next = word_array[i + 2];
		}

		System.out.println(answer);
	}
}