So it's going to be f of, well, if we're in the i-th rectangle, then we're going to take the left boundary is going to be x sub i minus 1 times delta x. And so right over here is a general way of considering, of thinking about approximating the area under a curve using rectangles where the height of the rectangles are defined by the left boundary. And this tells us it's the left boundary. And we see for each, if this is the i-th rectangle right over here, if this is rectangle i, then this right over here is x sub i minus 1, and this height right over here is f of x sub i minus 1. So that's all we did right over there, times delta x. And then you sum all of these from the first rectangle all the way to the n-th. So hopefully that makes you a little bit more comfortable with this notation.