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> t;
string s;
int n;
int fn(string const ss) {
  int occ = 0;
  string a = s;
  int i;
  for (i = 0; i < a.size(); i++) {
    a = s.substr(i, 2);
    if (a == ss) occ++;
  }
  return occ + 1;
}
bool comp(pair<string, int> a, pair<string, int> b) {
  return (a.second > b.second);
}
int main() {
  map<string, int>::iterator it;
  cin >> n;
  cin >> s;
  string ss;
  for (int i = 0; i < n - 1; i++) {
    ss = s.substr(i, 2);
    t[ss] = fn(ss);
  }
  pair<string, int> a = *t.begin();
  for (it = t.begin(); it != t.end(); it++) {
    if (comp(*it, a)) a = *it;
  }
  cout << a.first;
  return 0;
}
