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() {
  string s;
  int n;
  cin >> n;
  cin >> s;
  vector<int> v(n - 1, 1);
  string p, pom = s, d;
  vector<int>::iterator k = v.begin();
  for (int i = 1; i < n; ++i) {
    p = pom.substr(0, 2);
    pom = pom.substr(1);
    while (pom.length() > 1) {
      d = pom.substr(0, 2);
      pom = pom.substr(1);
      if (p.compare(s) == 0) ++*k;
    }
    ++k;
    pom = s.substr(i + 1);
  }
  int naj = 0, i = 0;
  for (k = v.begin(); k < v.end(); ++k) {
    if (naj < *k) {
      naj = *k;
      p = s.substr(i, i + 2);
    }
    ++i;
  }
  cout << p;
  return 0;
}
