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

#include <bits/stdc++.h>
using namespace std;
int main() {
  int p[50][50], n;
  char q[5000];
  while (~scanf("%d", &n)) {
    getchar();
    scanf("%s", q);
    memset(p, 0, sizeof(p));
    for (int i = 0; i < n - 1; i++) {
      p[q[i] - 'A'][q[i + 1] - 'A']++;
    }
    int maxa = 0;
    int maxb = 0;
    for (int i = 0; i < 26; i++) {
      for (int j = 0; j < 26; j++) {
        if (p[i][j] > p[maxa][maxb]) {
          maxa = i;
          maxb = j;
        }
      }
    }
    printf("%c%c\n", 'A' + maxa, 'A' + maxb);
  }
  return 0;
}
