#include <bits/stdc++.h>
using namespace std;
int tong[27];
int main() {
  string s;
  cin >> s;
  int n = s.length();
  s = ' ' + s;
  for (int i = 1; i <= n; i++) tong[s[i] - 'a' + 1]++;
  int cnt = 0;
  for (int i = 1; i <= 26; i++)
    if (tong[i] & 1) cnt++;
  if (cnt > 1) {
    int tot = 0;
    for (int i = 26; i >= 1; i--) {
      if (tong[i] & 1) tot++, tong[i]--;
      if (tot >= cnt / 2) break;
    }
    tot = 0;
    for (int i = 1; i <= 26; i++) {
      if (tong[i] & 1) tot++, tong[i]++;
      if (tot >= cnt / 2) break;
    }
  }
  string ans;
  char c;
  bool flag = false;
  for (int i = 1; i <= 26; i++) {
    if (tong[i] & 1) c = char(i - 1 + 'a'), flag = true;
    for (int j = 1; j <= tong[i] / 2; j++) ans += char(i - 1 + 'a');
  }
  cout << ans;
  if (flag) cout << c;
  reverse(ans.begin(), ans.end());
  cout << ans;
  return 0;
}
