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 final class B{
    public static void main(String[] args) {
        Scanner in=new Scanner(System.in);
        int N=in.nextInt();
        char[] cAry=in.next().toCharArray();
        HashMap<String,Integer> fmap=new HashMap<String,Integer>();
        for(int i=0;i<cAry.length-1;i++)
        {
            String key=new String(new char[]{cAry[i],cAry[i+1]});
            if(!fmap.containsKey(key))
            {
                fmap.put(key,1);
            }
            else
            {
                fmap.put(key,fmap.get(key)+1);
            }
        }
        int max=0;
        String maxKey="-1";
        for(String aKey:fmap.keySet())
        {
            int val=fmap.get(aKey);
            if(val>max)
            {
                max=val;
                maxKey=aKey;
            } 
        }
        System.out.println(maxKey);
    }
}