How to perform a logical operation (OR) on a number in SWI-Prolog?

Question

Prolog

My attempt:

?- A = 13, B = 1.
A = 13,
B = 1.

?- C is (A \/ B).
ERROR: is/2: Arguments are not sufficiently instantiated

What's wrong? It's like a built-in predicate, so it should work.
PS My goal is to quickly make an unpaired number out of a random number, for which I actually do such an operation. If the last bit is 1, then this number will be 100% unpaired, and this is what I need

Answer 1

1 ?- R is \/(13, 1).
R = 13.