Workflow - Calculating values from various fields

I have a question,

how can I calculate a value from several fields (sales_x,y,a,b) depending on filling values in brackets, for example:

((sales_x (value = 15) - sales_y (value = 5)) + ((sales_a (value = 25) - sales_b (value = -5)) = sales_z (value = 30) as a sum

BUT I NEED A CONDITION:

All sales from a given bracket (x, y) or (a, b) must be> 0

If one or both sales from parenthesis = 0 then it can not be taken to final summation, for example:

((sales_x (value = 0) - sales_y (value = 5)) + ((sales_a (value = 25) - sales_b (value = -5)) = sales_z (value = 20) as a sum only value sales_a and sales_b because sales_y=0