#include <bits/stdc++.h>   using namespace std;   const double PI = acos(-1.0);   int freq[26];   int main() {     string s;     cin >> s;     int mx = -1;     char g = s[0];     for (int i = 0; i < s.size(); i++) {       freq[s[i] -  a ]++;       if (s[i] >= g) {         mx = max(mx, freq[s[i] -  a ]);         g = s[i];       }     }     string d(freq[g -  a ], g);     cout << d << endl;     return 0;   }   