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