#include <bits/stdc++.h>
using namespace std;
int main(void) {
  ios_base::sync_with_stdio(0);
  int i, f, g;
  int charCount['z' + 10] = {0};
  char ch;
  int n = 0;
  while (cin >> ch) {
    charCount[ch]++;
    ++n;
  }
  f = 'z';
  g = 'a';
  while (true) {
    while (g <= 'z' and charCount[g] % 2 == 0) ++g;
    while (f >= 'a' and charCount[f] % 2 == 0) --f;
    if (g >= f) break;
    ++charCount[g];
    --charCount[f];
  }
  for (i = 'a'; i != 'z' + 1; i++) {
    for (g = 0; g != charCount[i] / 2; g++) cout << (char)i;
  }
  if (n % 2 == 1) cout << (char)f;
  for (i = 'z'; i >= 'a'; i--) {
    for (g = 0; g != charCount[i] / 2; g++) cout << (char)i;
  }
  return 0;
}
