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;
const int oo = INT_MAX, mod = 1e9 + 7;
const double PI = acos(-1), EPS = 1e-8;
int dr[]{-1, 1, 0, 0, 1, 1, -1, -1};
int dc[]{0, 0, 1, -1, 1, -1, 1, -1};
void run() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
}
int main() {
  run();
  int n;
  cin >> n;
  string s;
  cin >> s;
  int mx = 0;
  string ans;
  for (char ch = 'A'; ch <= 'Z'; ch++)
    for (char ch2 = 'A'; ch2 <= 'Z'; ch2++) {
      int cnt = 0;
      for (int i = 0; i + 1 < n; i++)
        if (s[i] == ch && s[i + 1] == ch2) cnt++, i++;
      if (cnt > mx) {
        ans = ch;
        ans += ch2;
        mx = cnt;
      }
    }
  return cout << ans, 0;
}
