#include <bits/stdc++.h>
int a[26];
char s[200010];
int main() {
  scanf("%s", s);
  memset(a, 0, sizeof(a));
  int len = strlen(s);
  for (int i = 0; s[i]; i++) a[s[i] - 'a']++;
  int l = 0, r = 25;
  while (1) {
    while (l < r && a[l] % 2 == 0) l++;
    while (l < r && a[r] % 2 == 0) r--;
    if (l >= r) break;
    a[l]++;
    a[r]--;
  }
  int k = 0;
  for (int i = 0; i < 26; i++)
    for (int j = 0; j < a[i] / 2; j++) s[k++] = 'a' + i;
  if (len % 2) s[k++] = 'a' + l;
  for (int i = 25; i >= 0; i--)
    for (int j = 0; j < a[i] / 2; j++) s[k++] = 'a' + i;
  s[k] = 0;
  printf("%s\n", s);
  return 0;
}
