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() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  ;
  int testCases = 1;
  while (testCases--) {
    int n;
    cin >> n;
    char arr[n];
    for (int i = 0; i < n; i++) {
      cin >> arr[i];
    }
    char ans[2];
    int max = INT_MIN;
    for (int i = 0; i < n - 1; i++) {
      int temp = 0;
      char brr[2];
      brr[0] = arr[i];
      brr[1] = arr[i + 1];
      for (int j = 0; j < n; j++) {
        if (arr[j] == brr[0] && arr[j + 1] == brr[1]) {
          temp++;
        }
      }
      if (temp > max) {
        max = temp;
        ans[0] = brr[0];
        ans[1] = brr[1];
      }
    }
    cout << ans[0] << ans[1] << '\n';
  }
  return 0;
}
