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(void) {
  int n;
  cin >> n;
  string str;
  cin >> str;
  map<string, int> m;
  if (str.length() == 2) {
    cout << str << endl;
  }
  for (size_t hi = 2; hi < str.length(); hi++) {
    string sstr = str.substr(hi - 2, 2);
    if (m.find(sstr) != m.end()) {
      m[sstr]++;
    } else {
      m.insert(make_pair(sstr, 1));
    }
  }
  pair<string, int> maxp("", 0);
  for (map<string, int>::iterator it = m.begin(); it != m.end(); it++) {
    if (it->second > maxp.second) {
      maxp = *it;
    }
  }
  cout << maxp.first << endl;
  return 0;
}
