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() {
  cin.tie(0);
  std::ios::sync_with_stdio(false);
  int n;
  cin >> n;
  string s;
  cin >> s;
  map<string, int> m;
  int i = 0;
  while (true) {
    string ss = s.substr(i, 2);
    m[ss]++;
    i++;
    if (i + 2 > s.size()) {
      break;
    }
  }
  map<string, int>::iterator iter;
  int sol = -1;
  string sss;
  for (iter = m.begin(); iter != m.end(); iter++) {
    if (sol < iter->second) {
      sol = iter->second;
      sss.clear();
      sss.append(iter->first);
    }
  }
  cout << sss << endl;
  return 0;
}
