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 cnt[5000];
void solve(int T) {
  int n;
  scanf("%d", &n);
  string s;
  cin >> s;
  for (int i = 0; i < n - 1; i++) {
    int num = (s[i] - 'A') + 100 * (s[i + 1] - 'A');
    cnt[num]++;
  }
  int ans = 0, a1, a2;
  for (int i = 1; i <= 4999; i++) {
    if (cnt[i] > ans) {
      ans = max(ans, cnt[i]);
      a1 = i % 100;
      a2 = (i - a1) / 100 + 'A';
      a1 += 'A';
    }
  }
  printf("%c%c", a1, a2);
}
signed main() {
  int t = 1;
  for (int i = 1; i <= t; i++) {
    solve(i);
  }
  return 0;
}
