Demonstrate that \int_{-\gamma} f(z)|dz|=\int_{\gamma} f(z)|dz| where \gamma is a piecewise smooth path and f is a function that is continuous on |\gamma|.

This proof seems like it should be very simple, but I am not sure it is really saying that it's just turning a path around to go in the opposite direction. Could someone please help me out? Thanks.