The derivative of this with respect to w is one. Or, if you multiply both sides times dw, you have du is equal to dw. And so, this left-hand side right over here, this integral could be rewritten as the integral of one over u du. We define this as u, and we just saw that du is the same thing as dw. So, this could be rewritten as du. And this is one of the basic integrals, so hopefully we've learned. And this is equal to the natural log of the absolute value of u.