I am working on finding codon usage bias and its determining factors in my species. Foe calculating P2 I have confusion.

P2 = (WWC+SSU)/(WWY+SSY)

where W = A or T, S = C or G and Y = C or T

I have values of A,T,G,C

Now when I put the value of W, I am confused whether to put A or T. I mean do I have to put A for first W and T for second W? like wise in others. or it is different