#include <bits/stdc++.h>
using namespace std;
int freq[500];
ifstream in("ordine.in");
ofstream out("ordine.out");
int main() {
  string s;
  in >> s;
  for (auto it : s) {
    freq[it]++;
  }
  for (int i = 1; i <= s.size(); i++) {
    char best_lett;
    bool ok = true;
    for (char c = 'z'; c >= 'a' && ok; c--) {
      if (freq[c]) best_lett = c;
      if (freq[c] == (s.size() - i + 1) / 2 + 1) {
        ok = false;
      }
    }
    out << best_lett;
    freq[best_lett]--;
  }
  return 0;
}