#include <bits/stdc++.h>
using namespace std;
const int N = 2e5 + 20, mod = (int)(1e9) + 10;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  string s;
  string c;
  bool b;
  char mx;
  map<char, int> m;
  cin >> s;
  s.size() % 2 == 0 ? b = 0 : b = 1;
  for (long long i = 0; i < s.size(); i++) {
    m[s[i]]++;
  }
  s.clear();
  for (auto i = m.begin(); i != m.end(); i++) {
    s.push_back(i->first);
  }
  for (long long i = 0; i < s.size(); i++) {
    if (m[s[i]] % 2 == 1) {
      c.push_back(s[i]);
      m[s[i]]++;
    }
  }
  if (c.size() == 1) {
    m[c[0]] = m[c[0]] - 2;
    mx = c[0];
  } else if (c.size() == 0)
    goto nikal;
  else {
    for (int i = c.size() - 1; i >= c.size() / 2; i--) {
      m[c[i]] -= 2;
      mx = c[c.size() / 2];
    }
  }
nikal:
  string o;
  for (long long i = 0; i < s.size(); i++) {
    for (int j = 1; j <= m[s[i]] / 2; j++) {
      o.push_back(s[i]);
    }
  }
  cout << o;
  if (b == 1) cout << mx;
  reverse(o.begin(), o.end());
  cout << o;
}
