#include <bits/stdc++.h>
using namespace std;
long long MOD = 1000000007;
int main() {
  string s;
  cin >> s;
  int n = s.length();
  vector<int> cnt(256, 0);
  for (int i = 0; i < (n); ++i) cnt[s[i]] += 1;
  for (char c = 'a', d = 'z'; c < d; ++c) {
    if (cnt[c] % 2 == 0) continue;
    while (c < d && cnt[d] % 2 == 0) d--;
    if (c < d) cnt[c] += 1, cnt[d] -= 1;
  }
  string ans;
  char tmp = 0;
  for (char c = 'a'; c <= 'z'; ++c) {
    for (int i = 0; i < (cnt[c] / 2); ++i) ans += c;
    if (cnt[c] % 2) tmp = c;
  }
  string ans1 = ans;
  reverse(ans1.begin(), ans1.end());
  if (tmp != 0) {
    ans += tmp;
    ans += ans1;
  } else
    ans += ans1;
  cout << ans << "\n";
  return 0;
}
