#include <bits/stdc++.h>
using namespace std;
int main() {
  vector<int> cnt(256, 0);
  string s;
  cin >> s;
  for (auto c : s) cnt[c]++;
  vector<int> odds;
  for (char i = 'a'; i <= 'z'; ++i)
    if (cnt[i] % 2 == 1) odds.push_back(i);
  int K = odds.size();
  for (int i = 0; i < K / 2; ++i) cnt[odds[i]]++, cnt[odds[K - i - 1]]--;
  for (char i = 'a'; i <= 'z'; ++i)
    for (int j = 0; j < cnt[i] / 2; ++j) cout << i;
  for (char i = 'a'; i <= 'z'; ++i)
    if (cnt[i] % 2 == 1) cout << i;
  for (char i = 'z'; i >= 'a'; --i)
    for (int j = 0; j < cnt[i] / 2; ++j) cout << i;
  cout << endl;
}
