The problem is that the moment you divided by two, it can no longer be an instance of Num.
Instances of Num include: Integers, Ints and Words. Once divided, you can no longer use that result in places where an Integer is required, because it won't be.
What you can do is use the `div` function which will round down to the nearest Integer value. Then it is an instance of Integral, which includes Integers, Ints, and Words (but floating point types)
You can also use `round`, `floor`, or `ceiling` to round your result to an appropriate integer after you've divided.