Really, as these become infinitely small and we have an infinite number of disks, so that our sum doesn't just approximate the volume. It actually is the volume at the limit. So to figure out the volume of this entire thing, we just have to evaluate this definite integral in terms of y. And so how do we do that? What's going to be equal to? Well, we could take the pi outside. It's going to be pi times the antiderivative of y, which is just y squared over 2, evaluated from 1 to 4, which is equal to pi times, well, you evaluate it at 4, you get 16 over 2.