Marginal PDF from Joint PDF of two Geometric Distribution

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$X$ and $Y$ are two independent Geometric distributions with the parameter '$p$'. $$Z = X+Y$$ $$W=X-Y$$ Now I have found the joint PDF as below:
$P_{Z,W}(i,j)=p^2(1-p)^{i-2}$ for $i>0$, $-i<j<i$ , $i+j$ is even here is my question: How can I find the marginal PDF for $W$? I know I should let $Z\to\infty$ somehow, but I'm not sure how to proceed doing so.