Add a negative operator(?) to a class in C#

I may not be using the right word here and that could be why I can't find the answer on my own. I have a + & - operator in my class, but I want to add a negative operator... I basicly what to be able to do this:

myMethod(myClass, -myClass);

If you need code examples to help me out let me know, but I think this should be pretty strait forward... Or that it can't be done.