title: The Worst Witch
context: The Worst Witch is a series of children's books written and illustrated by Jill Murphy. The series are primarily boarding school and fantasy stories, with eight books already published. The first one, The Worst Witch, was published in 1974 by Allison & Busby,[1] and the most recent one, First Prize for the Worst Witch, was published in 2018 by Puffin Books, the current publisher of all the series. The books have become some of the most successful titles on the Young Puffin paperback list and have sold more than 4 million copies.[2]
question: How many units has Worst Witch sold?
answers: 4 million