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.io.*;
public class math {
    private static int n;
    private static String [] s;
    private static String base;
    public static void main (String[] args ) throws Exception{
        input();
        String answer = "";
        int otvet = 0;
        for (int i = 0; i< math.n-2; i++){
            String wer = math.s[i] + math.s[i + 1];
            int k = 1;
            for (int g = i + 1; g < math.n-1; g++){
                String tyu = math.s[g] + math.s[g + 1];
                if (wer.equals(tyu)){
                    k++;
                }
            }
            if (k>otvet){
                otvet = k;
                answer = wer;
            }
        }
        System.out.println(answer);
    }
    private static void input() throws Exception{
        BufferedReader x = new BufferedReader(new InputStreamReader(System.in));
        math.n = Integer.parseInt(x.readLine());
        math.base = x.readLine();
        math.s = math.base.split("");
    }
}