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 Rec {
      static HashMap<String,Integer> h=new HashMap<>();
public static void main(String [] args){
Scanner sc=new Scanner(System.in);
int n=sc.nextInt();
sc.nextLine();
String s= sc.nextLine();
for(int i=0;i<n-1;i++){
   String k=s.substring(i,i+2);
   if(!h.containsKey(k))
      h.put(k,1);
   else{
         h.put(k,h.get(k)+1);
   }

}
int m=0;
String max=s;
for(String check:h.keySet()){
      if(h.get(check)>m){
            m=h.get(check);
            max=check;
      }
}
System.out.println(max);

}
}