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 i, j, n;
  cin >> n;
  string s, str1, str2, str3;
  cin >> s;
  vector<string> ss;
  for (i = 0; i < s.size() - 1; i++) {
    str1 = s[i];
    str2 = s[i + 1];
    str3 = str1 + str2;
    ss.push_back(str3);
  }
  int count[ss.size()];
  for (i = 0; i < ss.size(); i++) {
    for (j = 0; j < ss.size(); j++) {
      if (ss[i] == ss[j]) {
        count[i]++;
      }
    }
  }
  int max = count[0];
  int index = 0;
  for (i = 0; i < ss.size(); i++) {
    if (count[i] > max) {
      max = count[i];
      index = i;
    }
  }
  cout << ss[index] << endl;
}
