#include
#include  
using namespace std;
int main()
{
    string s = "aaaabbbcccddd";
  // unordered_set doesn't allow for duplicates
    unordered_set t(s.begin(), s.end()); 
    string u(t.begin(), t.end());
    sort(u.begin(), u.end());
    cout << u << endl;
    return 0;
}