#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  cin >> s;
  vector<int> cnt(26);
  for (int i = 0; i < (int)s.size(); ++i) {
    ++cnt[s[i] - 'a'];
  }
  int a = 0;
  int b = 25;
  while (a < b) {
    if (cnt[a] % 2 == 0) {
      ++a;
      continue;
    }
    if (cnt[b] % 2 == 0) {
      --b;
      continue;
    }
    ++cnt[a++];
    --cnt[b--];
  }
  int idx = 0;
  for (int c = 0; c < 26; ++c) {
    for (int i = 0; i < cnt[c] / 2; ++i) {
      s[idx++] = c + 'a';
    }
  }
  if (a == b && cnt[a] % 2 == 1) {
    s[idx++] = a + 'a';
  }
  for (int i = 0; 2 * i < (int)s.size(); ++i) {
    s[s.size() - 1 - i] = s[i];
  }
  cout << s << '\n';
}
