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 ans(int a, int b, int &count) {
  if (a > b) {
    return false;
  } else if (a == b) {
    return true;
  } else {
    if (ans(a * 2, b, count)) {
      count++;
      return true;
    }
    if (ans(a * 3, b, count)) {
      count++;
      return true;
    }
    return false;
  }
}
int main() {
  map<string, int> mp;
  int n;
  cin >> n;
  string s;
  cin >> s;
  for (int i = 0; i < s.size() - 1; i++) {
    string temp;
    temp += s[i];
    temp += s[i + 1];
    if (mp.find(temp) != mp.end()) {
      mp[temp]++;
    } else {
      mp.insert({temp, 1});
    }
  }
  string ans;
  int maxi = 0;
  for (auto i = mp.begin(); i != mp.end(); i++) {
    if (i->second > maxi) {
      maxi = i->second;
      ans = i->first;
    }
  }
  cout << ans << endl;
}
