I think you get the point here. Area of rectangle 3 all the way, plus all the way to the area of rectangle n. And so what are these going to be? Rectangle 1 is going to be its height, which is f of x0 or f of a. Either way, x0 and a are the same thing. So it's f of a times our delta x, times our width, our height times our width. So times delta, actually, let me write f of x0. I wanted to write f of x0 times delta x.