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.HashMap;
import java.util.Map;
import java.util.Scanner;

public class Test {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		int strLen = sc.nextInt();
		sc.nextLine();
		String str = sc.nextLine();
		Map<String, Integer> gramCount = new HashMap<>();
		
		int maxAppearance = 1;
		String maxAppearGram = str.substring(0, 2);
		for(int i = 0; i < strLen - 1; i++) {
			String curGram = str.substring(i, i + 2);
			int gramAppearance = gramCount.containsKey(curGram) ? gramCount.get(curGram) : 0;
			gramAppearance++;
			gramCount.put(str.substring(i, i + 2), gramAppearance);
			if(gramAppearance > maxAppearance) {
				maxAppearance = gramAppearance;
				maxAppearGram = curGram;
			}
		}
		
		System.out.println(maxAppearGram);
	}
}
