#include <bits/stdc++.h>
using namespace std;
int let[32];
int main() {
  string s, l = "", r = "", m = "";
  cin >> s;
  for (char c : s) let[c - 'a']++;
  int L = 0, R = 26;
  while (L != R) {
    while (L < R && let[L] % 2 == 0) L++;
    while (R > L && let[R] % 2 == 0) R--;
    if (L == R) break;
    let[L]++, let[R]--;
  }
  for (int i = 0; i < 28; i++) {
    if (let[i]) {
      if (let[i] & 1) m += i + 'a', let[i]--;
      for (int j = 0; j < let[i] / 2; j++) l += (i + 'a'), r += (i + 'a');
    }
  }
  sort(l.begin(), l.end());
  sort(r.begin(), r.end(), greater<char>());
  cout << l << m << r << endl;
}
