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;
map<string, int> cnt;
int main() {
  long long n;
  int l, mn = 0;
  string s, tmp;
  string sq("  ");
  cin >> n >> s;
  l = s.length();
  for (int i = 1; i < l; i++) {
    sq[0] = s[i - 1];
    sq[1] = s[i];
    if (sq.compare(0, 1, " ") && sq.compare(1, 1, " ")) {
      cnt[sq]++;
    }
  }
  map<string, int>::iterator it;
  for (it = cnt.begin(); it != cnt.end(); it++) {
    if (mn < it->second) {
      mn = it->second;
      tmp = it->first;
    }
  }
  cout << tmp << endl;
}
