#include <bits/stdc++.h>
using namespace std;
string s;
int cnt[26];
deque<int> deq;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cin >> s;
  for (int i = 0; i < s.size(); i++) cnt[s[i] - 'a']++;
  for (int i = 0; i < 26; i++)
    if (cnt[i] & 1) deq.push_back(i);
  while (deq.size() > 1) {
    int x = deq.front(), y = deq.back();
    cnt[y]--;
    cnt[x]++;
    deq.pop_back();
    deq.pop_front();
  }
  if (deq.size()) s[s.size() / 2] = deq.front() + 'a';
  int p = 0;
  for (int i = 0; i < 26; i++)
    for (int k = 0; k + 2 <= cnt[i]; k += 2)
      s[p] = s[s.size() - p - 1] = i + 'a', p++;
  cout << s << endl;
  return 0;
}
