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 B {

	public static void main(String[] args) {
	Scanner sc=new Scanner(System.in);
	int n=sc.nextInt();
	String s=sc.next();

	if(n!=s.length())
		return ;
	
	String a="";
	int c[]=new int[n-1];
	int x=0;
	
	
	for(int i=0;i<n-1;i++)
	{
		a=s.substring(i,i+2);
		
		for(int j=0;j<n-1;j++)
		{
			if(a==s.substring(j,j+2))
			c[x]++;
			
				
		}
		x++;
	}

	int max=Integer.MIN_VALUE;
	int index=0;
	for(int i=0;i<n-1;i++)
	{
		
		if(max<c[i])
		{	max=c[i];
		   index=i;
	    }
		
	}
	
	
	
	System.out.println(s.substring(index,index+2));
	}

}
