#include <bits/stdc++.h>
using namespace std;
int main() {
  char s[200005] = {0};
  int table[26] = {0};
  scanf(" %s", s);
  int len = strlen(s);
  for (int i = 0; i < len; i++) {
    table[s[i] - 'a']++;
  }
  int tmp = 26;
  for (int i = 0; i < tmp; i++) {
    if (table[i] % 2 == 1) {
      for (int k = tmp; k > i; k--) {
        if (table[k] % 2 == 1) {
          table[k]--;
          table[i]++;
          tmp = k - 1;
          break;
        }
      }
    }
  }
  int cnt = 0;
  for (int i = 0; i < 26; i++) {
    if (table[i] % 2 == 1) {
      s[len / 2] = 'a' + i;
    }
    for (int k = 0; k < table[i] / 2; k++) {
      s[cnt] = 'a' + i;
      s[len - 1 - cnt] = 'a' + i;
      cnt++;
    }
  }
  printf("%s\n", s);
}
