Make Text Negative Compared to the Background

I have searched for a while on this, but I can't seem to make it work. I have a white box moving up and down a black screen. I want text in front of the box. I want the parts of the text in front of the black background to be white, and the parts of the text in front of the white box to be black. I found this example, but, with my limited understanding of shaders, I can't make it work. Thanks.