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>
int solve() {
  int n;
  scanf("%d", &n);
  char s[128] = {};
  scanf("%s", s);
  int ans = 0;
  char ans_a = 0, ans_b = 0;
  for (int i = 0; i < n - 1; ++i) {
    char a = s[i];
    char b = s[i + 1];
    int cur = 0;
    for (int j = 0; j < n - 1; ++j) {
      if (a == s[j] && b == s[j + 1]) ++cur;
    }
    if (cur > ans) {
      ans = cur;
      ans_a = a;
      ans_b = b;
    }
  }
  printf("%c%c\n", ans_a, ans_b);
  return 0;
}
int main(int argc, char* argv[]) {
  solve();
  return 0;
}
