#include <bits/stdc++.h>
using namespace std;
int solve();
int main() { solve(); }
int solve() {
  string s;
  vector<int> cnt(26);
  cin >> s;
  int ncnt = 0;
  for (int i = 0; i < s.size(); ++i) ++cnt[s[i] - 'a'];
  for (int i = 0; i < 26; ++i) ncnt += (cnt[i] & 1);
  while (ncnt > 1) {
    for (int i = 25; i >= 0; --i) {
      for (int j = 0; j < i; ++j) {
        if ((cnt[i] & 1) && (cnt[j] & 1)) {
          ncnt -= 2;
          ++cnt[j];
          --cnt[i];
          break;
        }
      }
    }
  }
  for (int i = 0; i < 26; ++i) {
    for (int j = 0; j < cnt[i] / 2; ++j) {
      cout << (char)(i + 'a');
    }
  }
  if (ncnt) {
    for (int i = 0; i < 26; ++i) {
      if (cnt[i] & 1) cout << (char)(i + 'a');
    }
  }
  for (int i = 25; i >= 0; --i) {
    for (int j = 0; j < cnt[i] / 2; ++j) {
      cout << (char)(i + 'a');
    }
  }
  return 0;
}
