Reversing/inverting custom property driver

I’m working on a minecraft rig which will have armor, toggleable through custom properties. I’ve achieved this by attaching a driver to the mask modifier and setting the custom property to drive the mask visibility. I’ve been trying to work out a way to invert this, as right now 0=visible and 1=hidden in the property. Is there a way to make 0=hidden and 1=visible? I’ve tried setting the driver value to -1 but nothing seems to work. Inverting the mask makes the entire driver non-functional. Any help would be appreciated.

rig_stevePrivateV3.blend (1.61 MB)