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[] args) {
        ArrayList<String> maz = new ArrayList();
        int l = 0;
        Scanner sc = new Scanner(System.in);
        int test = sc.nextInt();
        String s = sc.next();
        for (int i = 0, i2 = 1; i2 < test; i++, i2++) {
            String a = s.substring(i, i2 + 1);
            if (s.substring(i2).contains(a)) {
                maz.add(l, a);
                l++;
            }
        }
        if (!(maz.isEmpty())) {

            int b = (s.split(maz.get(0))).length;
            for (int j = 0; j < maz.size(); j++) {
                if ((s.split(maz.get(j))).length >= b) {
                    b = (s.split(maz.get(j))).length;
                    System.out.println(maz.get(j));
                }
            }
        }
    }

}
