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;
void test_case() {}
bool found(string t, vector<string> s) {
  bool f = false;
  for (int i = 0; i < (int)s.size(); ++i) {
    if (s[i] == t) {
      f = true;
      break;
    }
  }
  return f;
}
int main() {
  int n;
  cin >> n;
  string p;
  cin >> p;
  vector<string> s;
  vector<string> a;
  for (int i = 0; i < n - 1; i++) {
    string t = "";
    for (int j = i; j < i + 2; ++j) {
      t += p[j];
    }
    a.push_back(t);
    if (!found(t, s)) {
      s.push_back(t);
    }
  }
  int maxm = 0;
  string ans = "?";
  for (string x : s) {
    int cnt = 0;
    for (int j = 0; j < n; ++j) {
      if (a[j] == x) {
        ++cnt;
      }
    }
    if (cnt > maxm) {
      maxm = cnt;
      ans = x;
    }
  }
  cout << ans << endl;
}
