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;
bool compare(pair<string, int> a, pair<string, int> b) {
  return a.first > b.first;
}
int main() {
  int n;
  string s;
  cin >> n >> s;
  map<string, int> m;
  int l = s.length();
  for (int i = 0; i < l - 1; i++) {
    stringstream ss;
    ss << s[i] << s[i + 1];
    string t = ss.str();
    if (m.find(t) != m.end())
      m[t]++;
    else
      m.insert(make_pair(t, 1));
  }
  int max1 = -1;
  string ans = "";
  for (pair<string, int> p : m) {
    if (p.second > max1) {
      max1 = p.second;
      ans = p.first;
    }
  }
  cout << ans;
  return 0;
}
