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, cnt = 0, maxn = 0;
  cin >> n;
  string a;
  cin >> a;
  char ans1 = a[0], ans2 = a[1];
  for (int i = 0, j = 1; i < a.length() - 1, j < a.length(); i++, j++) {
    cnt = 0;
    for (int e = i + 1, v = j + 1; e < a.length() - 1, v < a.length();
         e++, v++) {
      if (a[i] == a[e] && a[j] == a[v]) {
        cnt++;
      }
    }
    if (cnt > maxn) {
      ans1 = a[i];
      ans2 = a[j];
    }
  }
  cout << ans1 << ans2;
  return 0;
}
