Yep, what Graham suggested is what you need.
Basically, you need to know what variable carries information about primitive number. It's $PR - see Local Variables under hdox
/houdini/content/helpcards/sop/primitive.html
Then you need to find whether the primitive number is divisible by 2 or not.
For that you need the modulus function (%) - which gives you the remainder
of dividing one number by another number. If the remainder of division by 2 is 0, you know you're dealing with an even number.
Hence, the expression is $PR%2==0 (== checks whether a condition is true, = is the assignment operator, so make sure to use =