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 A_B_and_team_training {
	
	
	public static void main (String[] args) 
	{ 
		
		Scanner scn = new Scanner(System.in);
		int n=scn.nextInt();
		String str=scn.next();
		int max=Integer.MIN_VALUE;
		HashMap<String,Integer> hm=new HashMap<>();
		for(int i=0;i<n-1;i++){
			char a=str.charAt(i);
			char b=str.charAt(i+1);
		
			hm.put((char)a+""+(char)b+"", hm.getOrDefault(a+b+"", 0)+1);
			max=Math.max(max, hm.get((char)a+""+(char)b+""));
			
		}
		for(String key:hm.keySet()){
			if(max==hm.get(key)){
				System.out.println(key);
				break;
			}
		}
			
		}
	}
	
	
	

