#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  string s;
  cin >> s;
  int freq[26];
  memset(freq, 0, sizeof(freq));
  for (char c : s) freq[c - 'a']++;
  for (int i = 25; i >= 0; i--) {
    if (freq[i] % 2) {
      for (int j = 0; j < 26; j++) {
        if (i == j) continue;
        if (freq[j] % 2) {
          freq[j]++;
          freq[i]--;
          break;
        }
      }
    }
  }
  char ch = '#';
  string ans = "";
  for (int i = 0; i < 26; i++) {
    if (freq[i] % 2) ch = char(i + 'a');
    freq[i] /= 2;
    while (freq[i]) {
      ans.push_back(char(i + 'a'));
      freq[i]--;
    }
  }
  cout << ans;
  if (ch != '#') {
    cout << ch;
  }
  reverse(ans.begin(), ans.end());
  cout << ans << "\n";
  return 0;
}
