Charlie's Book Collection Problem

Charlie has a collection of books that he categorizes into mystery, adventure, and science fiction genres. Currently, he has 45 books in total: 12 are mystery books, and the remaining books are either adventure or science fiction.

He decides to donate some of his adventure books. If after donating, he has twice as many adventure books as science fiction books, and he originally had 18 adventure books, how many science fiction books does he have?