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() {
  char s[200];
  int num[200];
  int len;
  memset(s, 0, sizeof(s));
  memset(num, 0, sizeof(num));
  scanf("%d", &len);
  getchar();
  for (int i = 0; i < len; i++) {
    scanf("%c", &s[i]);
  }
  len = strlen(s);
  int max1 = 0;
  int madx;
  for (int i = 0; i < len; i++) {
    if ((s[i] + 1 == s[i + 1] || s[i] == s[i + 1] + 1 || s[i] == s[i + 1])) {
      num[i]++;
      for (int j = i + 1; j < len; j++) {
        if (s[j] == s[i] && s[j + 1] == s[i + 1]) {
          num[i]++;
        }
      }
      if (num[i] > max1) {
        madx = i;
        max1 = num[i];
      }
    }
  }
  printf("%c%c", s[madx], s[madx + 1]);
}
