#include <bits/stdc++.h>
using namespace std;
int main() {
  long long t = 1;
  while (t--) {
    string s;
    cin >> s;
    sort(s.begin(), s.end());
    long long n = s.size(), ans = 0;
    long long l = 0, r = n - 1;
    while (l <= r) {
      long long m = l + (r - l) / 2;
      if (s[m] == s[n - 1]) {
        if (s[m - 1] != s[n - 1] || m == 0) {
          ans = m;
          break;
        } else
          r = m - 1;
      } else {
        l = m + 1;
      }
    }
    cout << s.substr(ans) << endl;
  }
}
