Now, if we want the volume, we just have to multiply the area of the surface times the depth, times dy. So the volume of each of those disks is going to be pi y times dy. This gives you the volume of a disk. 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.