#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;
  long long int n = s.length(), i, j;
  vector<long long int> h(26);
  for (i = 0; i < n; i++) h[s[i] - 'a']++;
  vector<long long int> odd;
  for (i = 0; i < 26; i++) {
    if (h[i] % 2) odd.push_back(i);
  }
  long long int l = 0, r = (long long int)odd.size() - 1;
  while (l < r) {
    h[odd[l]]++;
    h[odd[r]]--;
    l++;
    r--;
  }
  long long int index =
      (((long long int)odd.size()) % 2 ? odd[((long long int)odd.size()) / 2]
                                       : 26);
  for (i = 0; i < 26; i++) {
    for (j = 0; j < h[i] / 2; j++) cout << (char)('a' + i);
  }
  if (index != 26) cout << char('a' + index);
  for (i = 25; i >= 0; i--) {
    if (i == index) {
      for (j = 0; j < h[i] / 2; j++) cout << (char)('a' + index);
      continue;
    }
    for (j = 0; j < (h[i] + 1) / 2; j++) cout << (char)('a' + i);
  }
  cout << "\n";
}
