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() {
  long long int n, i, j, k;
  string str;
  cin >> n;
  long long int chars[n];
  char c1, c2;
  cin >> str;
  for (i = 0; i < n; i++) chars[i] = str[i] - 'A';
  long long int matrix[26][26];
  memset(matrix, 0, sizeof matrix);
  for (i = 0; i < n - 1; i++) {
    matrix[chars[i]][chars[i + 1]]++;
  }
  k = 0;
  for (i = 0; i < 26; i++) {
    for (j = 0; j < 26; j++) {
      if (k < matrix[i][j]) {
        k = matrix[i][j];
        c1 = i + 65;
        c2 = j + 65;
      }
    }
  }
  cout << c1 << c2 << endl;
}
