For example, I understand that If I bet on red, my expected loss is (18/38 - 20/38) = -1/19 = -0.0526

However, I am confused when it comes to betting on a colour as well as numbers. For example, if I bet on red and the numbers 1, 2, 3, and 4, how would I go about calculating the expected loss then?

I figure it should be (18/38 - 20/38 + 20/38 - 18/38), but this would give 0, so this seems wrong.

Any help would be very much appreciated!