#include <bits/stdc++.h>
using namespace std;
string s;
long long check[1000];
vector<long long> p;
int main() {
  ios_base::sync_with_stdio(0);
  cin >> s;
  long long len = s.size();
  for (long long i = 0; i <= len - 1; i++) check[s[i] - 'a']++;
  for (long long i = 0; i <= 25; i++) {
    if (check[i] % 2) {
      p.push_back(i);
    }
  }
  long long len_p = p.size();
  if (len_p > 0) {
    for (long long i = 0; i <= (len_p - 1) / 2; i++) {
      check[p[i]]++;
      check[p[len_p - 1 - i]]--;
    }
  }
  string res = "";
  long long inx = -1;
  for (long long i = 0; i <= 25; i++)
    if (check[i] % 2) inx = i;
  for (long long i = 0; i <= 25; i++)
    for (long long j = 1; j <= check[i] / 2; j++) res += (i + 'a');
  string dao = res;
  reverse(dao.begin(), dao.end());
  string them = "";
  if (inx != -1) them += (inx + 'a');
  cout << res << them << dao;
  return 0;
}
