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() {
  int n;
  cin >> n;
  string s;
  cin >> s;
  vector<string> ts;
  vector<int> count;
  for (int i = 0; i < n - 1; i++) {
    string tsi = s.substr(i, 2);
    bool exists = false;
    for (int j = 0; j < ts.size(); j++) {
      if (ts[j].compare(tsi) == 0) {
        exists = true;
        count[j]++;
      }
    }
    if (exists == false) {
      ts.push_back(tsi);
      count.push_back(1);
    }
  }
  int max = 0;
  int maxInd = 0;
  for (int i = 0; i < count.size(); i++) {
    if (count[i] > max) {
      max = count[i];
      maxInd = i;
    }
  }
  cout << ts[maxInd] << endl;
  return 0;
}
