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;
string s;
int k['Z' - 'A' + 1]['Z' - 'A' + 1];
int mx_v = 0, mx_ix = 0, mx_iy = 0;
int main() {
  for (int i = 0; i < 'Z' - 'A' + 1; i++)
    for (int j = 0; j < 'Z' - 'A' + 1; j++) k[i][j] = 0;
  getline(cin, s);
  for (int i = 0; i < s.length() - 1; i++) k[s[i] - 'A'][s[i + 1] - 'A'] += 1;
  for (int i = 0; i < 'Z' - 'A' + 1; i++)
    for (int j = 0; j < 'Z' - 'A' + 1; j++)
      if (k[i][j] > mx_v) {
        mx_v = k[i][j];
        mx_ix = i;
        mx_iy = j;
      }
  cout << (char)(mx_ix + 'A') << (char)(mx_iy + 'A');
  return 0;
}
