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() {
  int n;
  cin >> n;
  string s;
  cin >> s;
  map<string, int> m;
  for (int i = 0; i < n - 1; i++)
    if (abs(s[i] - s[i + 1]) < 2) {
      string semp;
      semp += s[i];
      semp += s[i + 1];
      m[semp]++;
    }
  string sol;
  int mx = INT_MIN;
  for (auto u : m)
    if (u.second > mx) {
      mx = u.second;
      sol = u.first;
    }
  cout << sol << endl;
  return 0;
}
