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 main() {
  int n, max, i, rem1, rem2;
  while (scanf("%d", &n) == 1) {
    int cnt[92] = {0};
    char str[n + 2];
    scanf("%s", str);
    max = 0;
    for (i = 0; i < n - 1; i++) {
      if (str[i] == str[i + 1]) {
        cnt[str[i]]++;
        cnt[str[i + 1]]++;
      } else if (str[i] > str[i + 1]) {
        if (str[i] - str[i + 1] == 1) {
          cnt[str[i]]++;
          cnt[str[i + 1]]++;
        }
      } else if (str[i] == 90 && str[i + 1] == 65) {
        cnt[str[i]]++;
        cnt[str[i + 1]]++;
      } else if (str[i] == 65 && str[i + 1] == 90) {
        cnt[str[i]]++;
        cnt[str[i + 1]]++;
      } else {
        if (str[i + 1] - str[i] == 1) {
          cnt[str[i]]++;
          cnt[str[i + 1]];
        } else {
          continue;
        }
      }
      if (cnt[str[i]] > max && cnt[str[i + 1]] > max) {
        max = cnt[str[i]];
        rem1 = (int)str[i];
        rem2 = (int)str[i + 1];
      }
    }
    printf("%c%c\n", rem1, rem2);
  }
  return 0;
}
