#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  string s;
  cin >> s;
  string ans1 = "", ans2 = "";
  map<char, int> m;
  for (int i = 0; i < s.size(); i++) {
    m[s[i]]++;
  }
  vector<char> v;
  long long cnt = 0;
  for (char i = 'a'; i <= 'z'; i++) {
    if (m[i] % 2 == 0) {
      for (int j = 1; j <= m[i] / 2; j++) {
        ans2 = ans2 + i;
      }
    } else if (m[i] % 2 == 1) {
      for (int j = 1; j <= m[i] / 2; j++) {
        ans2 = ans2 + i;
      }
      ans1 = ans1 + i;
      cnt++;
    }
  }
  string tmp = "";
  for (int i = 0; i < ans1.size() - cnt / 2; i++) {
    tmp += ans1[i];
  }
  ans1 = "";
  string ans = "";
  if (cnt % 2 == 0) {
    ans = ans2 + tmp;
    sort(ans.begin(), ans.end());
    for (int i = ans.size() - 1; i >= 0; i--) ans1 += ans[i];
  } else {
    for (int i = tmp.size() - 2; i >= 0; i--) ans1 += tmp[i];
    ans = ans1 + ans2;
    ans1 = "";
    sort(ans.begin(), ans.end());
    for (int i = ans.size() - 1; i >= 0; i--) ans1 += ans[i];
    ans = ans + tmp[tmp.size() - 1];
  }
  cout << ans + ans1;
}
