#include <bits/stdc++.h>
using namespace std;
signed main() {
  ios_base::sync_with_stdio(false);
  string s;
  cin >> s;
  map<char, long long> m;
  for (long long i = 0; i < s.size(); i++) m[s[i]]++;
  long long ev = 0, od = 0;
  vector<char> v;
  for (auto it = m.begin(); it != m.end(); it++) {
    if (it->second % 2 == 0)
      ev++;
    else {
      od++;
      v.push_back(it->first);
    }
  }
  if (od > 1) {
    sort(v.begin(), v.end());
    for (long long i = 0; i < od / 2; i++) {
      m[v[i]]++;
      m[v[od - i - 1]]--;
    }
  }
  char mid = 'A';
  string ans;
  for (auto it = m.begin(); it != m.end(); it++) {
    for (long long i = 0; i < it->second / 2; i++) ans.push_back(it->first);
    if (it->second % 2 != 0) mid = it->first;
  }
  string back = ans;
  reverse(back.begin(), back.end());
  if (mid != 'A') ans += mid;
  ans += back;
  cout << ans;
}
