#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(0), cin.tie(0);
  string s;
  cin >> s;
  sort(s.begin(), s.end());
  reverse(s.begin(), s.end());
  cout << s[0];
  for (long long int(i) = (1); (i) < (s.length()); (i)++) {
    if (s[i] == s[i - 1])
      cout << s[i];
    else
      break;
  }
  cout << "\n";
}
