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.*;
        import java.io.*;
        public class abc
        {
            public static void main(String args[])
            {
                String a="",b="",c="";
            int max=0,count=0,x,y;
            Scanner sc=new Scanner(System.in);
           int l=sc.nextInt();
	    String s=sc.next();
            for(x=0;x<(l-1);x++)
            {
                b=s.substring(x,x+2);
                count=0;
                for(y=0;y<(l-1);y++)
                {
                    c=s.substring(y,y+2);
                    if(b.equals(c))count++;
                }
                if(count>max)
		{

                a=b;
		max=count;
		}
            }
            System.out.println(a);
            
        }
        }
