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;
  int counter = 0;
  cin >> n;
  string c;
  cin >> c;
  if (n == 2) {
    cout << c;
    return 0;
  }
  if (n == 4) {
    cout << c[1] << c[2];
    return 0;
  }
  char k1;
  char k2;
  char mas1[n];
  char mas2[n];
  int mas[n - 1];
  for (int i = 0; i < n - 1; i++) {
    for (int j = 0; j < n - 1; j++) {
      if (c[i] == c[j] && c[i + 1] == c[j + 1]) {
        counter++;
        k1 = c[i];
        k2 = c[j];
      }
    }
    if (counter != 0) {
      mas1[i] = k1;
      mas2[i] = k2;
      mas[i] = counter;
    }
    counter = 0;
  }
  int maxi = mas[0];
  int ind = 0;
  for (int i = 0; i < n - 1; i++) {
    if (mas[i] > maxi) {
      maxi = mas[i];
      ind = i + 1;
    }
  }
  cout << mas1[ind] << mas2[ind];
}
