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 Solution {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        
        int n = sc.nextInt();
        sc.nextLine();
        
        String s = sc.next();
        
        HashMap<String, Integer> hMap = new HashMap<String, Integer>();
        
        for(int i=0; i<s.length()-1; i++) {
            String twoGram = s.substring(i, i+2);
            
            if(hMap.containsKey(twoGram)) {
                int count = hMap.get(twoGram);
                hMap.remove(twoGram);
                hMap.put(twoGram, ++count);
            }
            else {
                hMap.put(twoGram, 1);    
            }
        }
        
        Iterator hmIterator = hMap.entrySet().iterator();
        
        int count = Integer.MIN_VALUE;
        String answer = "";
        while(hmIterator.hasNext()) {
            Map.Entry mapElement = (Map.Entry)hmIterator.next();
            if(count < (Integer)mapElement.getValue()) {
                answer = (String)mapElement.getKey();
            }
        }
        
        System.out.println(answer);
    }
}