#include <bits/stdc++.h>
using namespace std;
string s;
char c;
string fin = "", ans = "";
int fre[26], flag;
int main() {
  cin >> s;
  for (int i = 0; i < (int)s.length(); ++i) {
    fre[(int)(s[i] - 'a')]++;
  }
  deque<int> q;
  for (int i = 0; i < 26; ++i) {
    if (fre[i] & 1) q.push_back(i);
  }
  while (q.size() > 1) {
    fre[q.front()]++;
    fre[q.back()]--;
    q.pop_front();
    q.pop_back();
  }
  for (int i = 0; i < 26; ++i) {
    for (int j = 0; j < fre[i] / 2; ++j) {
      ans += ('a' + i);
    }
    if (fre[i] & 1) {
      flag = 1;
      c = ('a' + i);
    }
  }
  fin = ans;
  reverse(ans.begin(), ans.end());
  if (flag) fin += c;
  fin += ans;
  cout << fin << '\n';
}
