#include <bits/stdc++.h>
#pragma comment(linker, "/STACK:64000000")
using namespace std;
const double eps = (double)(1e-9);
int c['z' + 1];
int main() {
  string s, x;
  int i, j;
  x = "a";
  cin >> s;
  for (i = 0; i < s.length(); i++) {
    c[s[i]]++;
  }
  i = 'a';
  j = 'z';
  while (1) {
    while (i < j && c[i] % 2 == 0) i++;
    while (i < j && c[j] % 2 == 0) j--;
    if (i >= j) break;
    c[i]++;
    c[j]--;
  }
  s = "";
  for (i = 'a'; i <= 'z'; i++) {
    if (c[i] % 2) {
      x[0] = i;
      c[i]--;
      s = x;
    }
  }
  for (i = 'z'; i >= 'a'; i--) {
    x[0] = i;
    for (j = 0; j < c[i] / 2; j++) s = x + s + x;
  }
  cout << s;
  return 0;
}
