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 arg[]) {
        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());
        String str = sc.next();
        Map<String, Integer> map = new HashMap<>();
        for(int i = 0; i < n - 1; ++i){
            String sub = str.substring(i, i + 2);
            if(!map.containsKey(sub)){
                map.put(sub, 0);
            }
            map.put(sub, map.get(sub) + 1);
        }
        System.out.println(map.entrySet().stream().sorted((a, b) -> b.getValue() - a.getValue()).map(pair -> pair.getKey()).findFirst().orElse(""));
   }
}
