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 Main {
	public static void main(String[] args) {
		
		Scanner sc=new Scanner(System.in);
		try{
		String data=sc.next();
		String limit=sc.next();
		ArrayList<String>al=new ArrayList<String>();
		for(int i=0;i<data.length();i++)
		{
			String sub=data.substring(i,i+2);
			al.add(sub);
			
				
			
		}
		int max=Collections.frequency(al, al.get(0));
		String output=al.get(0);
		for(int i=1;i<al.size();i++)
			
		{
			String s=al.get(i);
			if(max<Collections.frequency(al, s))
			{
				max=Collections.frequency(al, s);
				output=s;
			}
			
		}
		//System.out.print(al);
		System.out.println(output);
		}
		catch(Exception e)
		{
		    
		}
	
	}
}