#include <bits/stdc++.h>
using namespace std;
const int N = 1e5 + 10;
const long long inf = 1e18 + 1;
string st;
int freq[26];
int main() {
  cin >> st;
  for (int i = 0; i < st.size(); i++) ++freq[st[i] - 'a'];
  for (int i = 0; i < 26; i++) {
    if (freq[i] % 2 != 0)
      for (int j = 25; j > i; j--) {
        if (freq[j] % 2 != 0) {
          freq[j]--;
          freq[i]++;
          break;
        }
      }
  }
  string ans = "";
  bool bl = false;
  char ch;
  for (int i = 0; i < 26; i++) {
    if (freq[i] % 2 != 0) {
      bl = true;
      ch = (char)i + 'a';
    }
    for (int j = 0; j < freq[i] / 2; j++) ans += (char)i + 'a';
  }
  sort(ans.begin(), ans.end());
  cout << ans;
  reverse(ans.begin(), ans.end());
  if (bl) cout << ch;
  cout << ans << endl;
  return 0;
}
