#include <bits/stdc++.h>
using namespace std;
const int MAX_ALPHA = 30;
int a[MAX_ALPHA];
int main() {
  string s;
  getline(cin, s);
  int n = (int)s.length();
  for (int i = 0; i < n; i++) a[s[i] - 'a']++;
  int p = -1;
  for (int i = 0; i < MAX_ALPHA; i++)
    if (a[i] % 2 != 0) {
      int j = MAX_ALPHA - 1;
      while (j >= i && a[j] % 2 == 0) j--;
      if (i != j) {
        a[i]++;
        a[j]--;
      } else
        p = i;
    }
  for (int i = 0; i < MAX_ALPHA; i++)
    for (int j = 0; j < a[i] / 2; j++) cout << (char)(i + 'a');
  if (p != -1) cout << (char)(p + 'a');
  for (int i = MAX_ALPHA - 1; i >= 0; i--)
    for (int j = 0; j < a[i] / 2; j++) cout << (char)(i + 'a');
  return 0;
}
