If you need two random choices, start by not caring about the actual values. You need a random number that returns exactly one of two numbers. random(n) returns a number between 0 and n-1. So random(2) will return 0 or 1
Now multiply that with the -180 and you get either 0 or -180 (0 * -180 = 0, 1 * -180 = -180
The expression should look somewhat like that:
random(2) * -180
EDIT: I'd say, this thread better belongs to the help section ;)