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 n;
string s;
void in() {
  cin >> n;
  cin >> s;
}
bool check(char s) {
  if (s >= 'A' && s <= 'Z') {
    return true;
  }
  return false;
}
void sol() {
  string result;
  for (int i = 0; i < n - 1; i++) {
    if (check(s[i]) == true && check(s[i + 1]) == true) {
      result = s.substr(i, i + 1);
    }
  }
  cout << result;
}
int main(void) {
  ios::sync_with_stdio(0);
  cin.tie(0);
  in();
  sol();
  return 0;
}
