#include <bits/stdc++.h>
using namespace std;
int c[27];
int main() {
  string s;
  cin >> s;
  for (int i = 0; i < s.length(); i++) {
    int idx = s[i] - 'a';
    c[idx]++;
  }
  int ch = 0;
  if (s.length() % 2) ch = 1;
  int p = 26;
  int i = 0, j = 25;
  while (i < j && i < 26 && j >= 0) {
    if (c[i] % 2 == 0)
      i++;
    else if (c[j] % 2 == 0)
      j--;
    else {
      c[i++]++;
      c[j++]--;
    }
  }
  if (ch == 1) {
    for (int i = 25; i >= 0; i--) {
      if (c[i] % 2) {
        p = i;
        break;
      }
    }
  }
  string ans = "";
  for (int i = 0; i < 26; i++) {
    int x = c[i] / 2;
    for (int j = 0; j < x; j++) ans += char(i + 'a');
  }
  string ans2 = ans;
  reverse(ans2.begin(), ans2.end());
  if (ch == 1) ans += char(p + 'a');
  ans += ans2;
  cout << ans;
}
