BLOOMINGTON, Ind. (AP) — Indiana University students have saved an estimated $3.5 million during the 2016-17 school year by using electronic textbooks.

The Bloomington Herald-Times reports that Indiana University’s office of the vice president for information technology, which runs the university’s electronic textbook program, estimates that more than 40,000 students across all campuses used at least one electronic textbook last year.

The use of electronic textbooks is not mandatory and individual faculty members decide if they want to use them for a course. Students who sign up for a course with an electronic textbook are automatically charged for the book on their bursar account.

Stacy Morrone, the university’s associate vice president for learning technologies, says the electronic textbook program was fully launched in 2012 and now has more than 88,000 titles available.