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;
const long long N = 1e5 + 5;
long long n, m, i, j, frq[30][30];
string s;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  if (0)
    ;
  cin >> n >> s;
  for (i = 0; i < n - 1; i++) {
    frq[s[i] - 'A'][s[i + 1] - 'A']++;
  }
  string ans = "";
  long long mxfrq = 0;
  for (i = 0; i < 30; i++) {
    for (j = 0; j < 30; j++) {
      if (frq[i][j] > mxfrq) {
        ans = "";
        ans += (char)('A' + i);
        ans += (char)('A' + j);
        mxfrq = frq[i][j];
      }
    }
  }
  cout << ans << endl;
}
