0%, sweetheart. The two periods are a dead giveaway.
If you can't believe it, go see your doctor and have a sperm test done. If some errant sperm are still getting through, you could pay for her to have a prenatal DNA test, that is, if you don't think the periods were really periods. But you might look like an idiot asking her to do it, since there is no reason to assume they weren't periods. It sounds like she got pregnant weeks and weeks after the sex with you.
If you're interested in the girl and kind of wish the baby was from you, go ahead and date her and see where things go. If you are bothered by the fact that she is pregnant by someone else, just handle it as if she is a woman who already had a child before you knew her. The issues will be about the same as if she had a toddler at her side the day you met.