Well, it's going to be the area of the first rectangle. So let me write this down. So it's going to be rectangle 1, so the area of rectangle 1, so rectangle 1 plus the area of rectangle 2, plus the area of rectangle 2, plus the area of rectangle 3. 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.