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, i, j, ct = 0, m = -1, p = -1;
  string s, s1, s2;
  vector<string> vec;
  cin >> n;
  cin >> s;
  if (n > 2) {
    for (i = 0; i < n; i++) {
      s1 = s.substr(i, 2);
      vec.push_back(s1);
    }
    sort(vec.begin(), vec.end());
    for (i = 0; i < vec.size(); i++) {
      if (vec[i] == vec[i + 1]) {
        ct++;
        m = max(m, ct);
        if (m > p) {
          p = m;
          s2 = vec[i];
        }
      } else {
        ct = 0;
      }
    }
    if (m == -1)
      cout << vec[0] << endl;
    else
      cout << s2 << endl;
  } else {
    cout << s << endl;
  }
  return 0;
}
