#include <bits/stdc++.h>

#define rep(i, j, k) for (int i = (j); i <= (k); ++i)
#define per(i, j, k) for (int i = (j); i >= (k); --i)
#define SZ(v) int((v).size())
#define ALL(v) (v).begin(),(v).end()
#define fi first
#define se second
using ll = long long;
using pii = std::pair<int, int>;
using pll = std::pair<ll, ll>;

template<class T> void chkmn(T &x, T y) { if (y < x) x = y; }
template<class T> void chkmx(T &x, T y) { if (y > x) x = y; }

using namespace std;

string str;

multiset<string> s;

int main() {
  cin.tie(nullptr) -> ios::sync_with_stdio(false);
  cin >> str;
  for (char ch : str) s.insert(""s + ch);
  while (SZ(s) > 1) {
    string x = *s.begin(), y = *s.rbegin();
    s.erase(s.find(x)), s.erase(s.find(y));
    s.insert(x + y);
  }
  cout << (*s.begin()) << '\n';
}