#include <bits/stdc++.h>
const int A = 26, MN = 2 * 1000 * 100 + 42;
char str[MN];
int cou[MN];
int main() {
  scanf("%s", str);
  for (int i = 0; str[i]; ++i) cou[str[i] - 'a']++;
  int odd = 0;
  for (int i = 0; i < A; ++i)
    if (cou[i] % 2) odd++;
  int ch = 0;
  for (int i = 0; i < A; ++i)
    if (cou[i] % 2) {
      if (ch * 2 < odd - 1)
        cou[i]++;
      else if (ch * 2 > odd - 1)
        cou[i]--;
      ch++;
    }
  for (int i = 0; i < A; ++i)
    for (int k = 0; k < cou[i] / 2; ++k) putchar('a' + i);
  for (int i = 0; i < A; ++i)
    if (cou[i] % 2) putchar('a' + i);
  for (int i = A - 1; i >= 0; --i)
    for (int k = 0; k < cou[i] / 2; ++k) putchar('a' + i);
}
