I should say a local minimum point. And so if it's at a local minimum or maximum point, really, the derivative of w is going to be zero. So let's see, at what t do we get a zero derivative? So w prime of t. Well, the derivative of a constant with respect to t is zero. And the derivative of this with respect to t, and this comes straight out of the fundamental theorem of calculus, this is going to be, this is going to be r of t minus d of t. Where once again, this is a function. We have t as the upper bound. And so we have whatever here that we were integrating, but it's now going to be a function of t. And so in order for this to be equal to zero, we need to figure out when does r of t minus d of t equal zero.