#include <bits/stdc++.h>
using namespace std;
const int MOD = 1000000007;
const int INF = int(1e9);
int main() {
  ios_base::sync_with_stdio(false);
  string s;
  cin >> s;
  vector<int> cnt(26);
  for (char x : s) {
    cnt[x - 'a']++;
  }
  vector<int> odd;
  for (int i = 0; i < 26; i++) {
    if (cnt[i] % 2) {
      odd.push_back(i);
    }
  }
  int l = 0, r = odd.size() - 1;
  while (l < r) {
    cnt[odd[l]]++;
    l++;
    cnt[odd[r]]--;
    r--;
  }
  int m = -1;
  string a;
  for (int i = 0; i < 26; i++) {
    if (cnt[i] % 2 == 1) {
      m = i;
    }
    cnt[i] /= 2;
  }
  for (int i = 0; i < 26; i++) {
    for (int j = 0; j < cnt[i]; j++) {
      a += char(i + 'a');
    }
  }
  string c = a;
  reverse(c.begin(), c.end());
  string res;
  if (m != -1) {
    res += a + char(m + 'a') + c;
  } else {
    res += a + c;
  }
  cout << res << "\n";
  return 0;
}
