#include <bits/stdc++.h>
using namespace std;
int c[30];
int fi;
void print(int i) {
  if (i > 25) {
    if (c[fi] & 1) printf("%c", fi + 'a');
    return;
  }
  for (int(j) = (0); (j) < (c[i] / 2); ++(j)) printf("%c", i + 'a');
  print(i + 1);
  for (int(j) = (0); (j) < (c[i] / 2); ++(j)) printf("%c", i + 'a');
}
char str[300000];
int main() {
  memset((c), (0), sizeof(c));
  scanf("%s", str);
  int n = strlen(str);
  for (int(i) = (0); (i) < (n); ++(i)) c[str[i] - 'a']++;
  int qnt = 0;
  for (int(i) = (0); (i) < (26); ++(i))
    if (c[i] & 1) qnt++;
  fi = 0;
  for (int i = 25, j = 0; i > j;) {
    while (i > j && !(c[i] & 1)) i--;
    while (i > j && !(c[j] & 1)) j++;
    if (i <= j) {
      if (qnt & 1) fi = i;
      break;
    }
    c[j]++, c[i]--;
  }
  print(0);
  puts("");
}
