#include <bits/stdc++.h>
using namespace std;
int32_t main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  string s;
  cin >> s;
  vector<long long> cnt(26, 0);
  for (char c : s) cnt[c - 'a']++;
  for (long long i = 0, j = 25; i < j;) {
    if (cnt[i] % 2 == 0)
      i++;
    else if (cnt[j] % 2 == 0)
      j--;
    else {
      cnt[i]++, cnt[j]--;
      i++, j--;
    }
  }
  long long prev = -1;
  for (long long i = 0; i < 26; i++)
    if (cnt[i] & 1) prev = i;
  vector<char> res(s.size() / 2);
  long long idx = 0;
  for (long long i = 0; i < 26; i++)
    while (cnt[i] > 1) {
      res[idx++] = 'a' + i;
      cnt[i] -= 2;
    }
  for (char c : res) cout << c;
  if (prev != -1) cout << (char)(prev + 'a');
  reverse(res.begin(), res.end());
  for (char c : res) cout << c;
  return 0;
}
