#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  for (string s; cin >> s;) {
    int freq[128] = {};
    for (char ch : s) {
      ++freq[ch];
    }
    char prev = 0;
    for (char i = 'a', j = 'z'; i < j;) {
      while (freq[i] % 2 == 0 && i <= 'z') ++i;
      while (freq[j] % 2 == 0 && j >= 'a') --j;
      if (i < j) {
        ++freq[i], --freq[j];
      }
      if (i == j) {
        prev = i;
      }
    }
    s.clear();
    for (char ch = 'a'; ch <= 'z'; ++ch) {
      s.append((ch == prev ? freq[ch] - 1 : freq[ch]) / 2, ch);
    }
    cout << s;
    if (prev) cout << prev;
    reverse(s.begin(), s.end());
    cout << s << endl;
  }
}
