Now, if we want the volume of this entire thing, we just have to sum all of these disks for all the y values between y is equal to 1 and y is equal to 4. So let's do that. So we just take the definite integral from y is equal to 1 to y equals 4. And just as a reminder, definite integral is a very special type of sum. We're summing up all of these things, but we're taking the limit of that sum as these dy's get shorter, or gets, I guess, squatter and squatter, smaller and smaller. And we have a larger and larger number of these disks. Really, as these become infinitely small and we have an infinite number of disks, so that our sum doesn't just approximate the volume.