#include <bits/stdc++.h>
using namespace std;
const int MAX = 2e5 + 10;
char s[MAX];
int n[26];
int main() {
  scanf("%s", s);
  int nl = strlen(s);
  for (int i = 0; i < nl; i++) n[s[i] - 'a']++;
  int l = 0, r = 25;
  while (1) {
    while (n[r] % 2 == 0) r--;
    while (n[l] % 2 == 0) l++;
    if (l >= r) break;
    n[r]--, n[l]++;
  }
  l = 0;
  for (int i = 0, j = nl - 1; i < j; i++, j--) {
    while (n[l] < 2) l++;
    n[l] -= 2;
    s[i] = s[j] = l + 'a';
  }
  if (nl & 1) {
    l = 0;
    while (!n[l]) l++;
    s[nl / 2] = l + 'a';
  }
  printf("%s\n", s);
  return 0;
}
