#include <bits/stdc++.h>
using namespace std;
char s[200005];
int alph[26];
int main() {
  scanf("%s", &s);
  for (int i = 0; s[i]; i++) alph[s[i] - 'a']++;
  for (int i = 0; i < 26; i++)
    for (int j = 26 - 1; j > i; j--)
      if (alph[i] & 1)
        if (alph[j] & 1) alph[i]++, alph[j]--;
  int odd = -1;
  for (int i = 0; i < 26; i++)
    if (alph[i] & 1) odd = i;
  for (int i = 0; i < 26; i++)
    for (int j = 1; j <= alph[i] / 2; j++) printf("%c", i + 'a');
  if (odd != -1) printf("%c", odd + 'a');
  for (int i = 26 - 1; i >= 0; i--)
    for (int j = 1; j <= alph[i] / 2; j++) printf("%c", i + 'a');
  return 0;
}
