12. All 45 students in Class 3(1) have signed up for the school sports meet. Apart from the tug-of-war competition where the entire class participates, each student must participate in at least one of the other three events. It is known that 39 students in the class have signed up for the kick shuttlecock competition, and 28 students have signed up for the basketball shooting. How many students have signed up for all three events?