IMHO, the bug is about what set to print if there are more than one sets sattisfying the condition of the problem.
In the problem it is said: "...you should print to the output only one (preferably your favorite) of them."
But after a few tries I came to a conclusion that not "favourite", but SMALLEST set is required... Strange, huh?
After I modified my program to output smallest possible set it got AC. Other modifications got WA.