Hello,

Your mistake is that you're not considering X as a random variable.

What you can compute is P(all items marked|X=x)=s^x

But if you want the probability, not the conditional one, just remember that E[X]=E[E[X|Y]] (and that a probability is an expectation).

Thus P(all items marked)=E[P(all items marked|X)]=E[s^X]