# include <bits/stdc++.h>

namespace kh {
  using namespace std;
  void main() {
    string input;
    cin >> input;
    map<string, int> s;
    for (char c : input)
      ++s[string(1, c)];
    while (s.size() > 1) {
      auto i(s.begin()), j(prev(s.end()));
      string const& s0(next(i, 0)->first);
      string const& s1(next(i, 1)->first);
      if (i->second == 1) {
        if (s0.size() >= s1.size()) break;
        for (int k(s0.size() - 1); k >= 0; --k)
          if (s0[k] != s1[k]) goto line27;
      }
      string cat(i->first + j->first);
      int cnt(min(i->second, j->second));
      if (!(i->second -= cnt)) s.erase(i);
      if (!(j->second -= cnt)) s.erase(j);
      s[cat] += cnt;
    }
line27:
    string ans;
    while (s.begin()->second) {
      ans += s.begin()->first;
      --s.begin()->second;
    }
    for (auto i(s.rbegin()); i != s.rend(); ++i)
      for (int j(0); j != i->second; ++j)
        ans += i->first;
    cout << ans << endl;
  }
}

int main() { kh::main(); }