To you Math majors out there....

So I wear contact lenses that have to be changed every two weeks (and I take them out every night). Today was the day that I made the swap. Based on past experiences, I don't throw out the old pair until I know everything is okay with the new pair (if there's a tear, for example, I still have the old pair to fall back on).

Anyway, I just went to put my new contacts back in the canister, and afterwards I immediately realized that I never threw out the old set. So I reopened both lids and kept the contact that seemed to be "on top," and tossed the other one.

So here's my question: What are the odds that both of the lenses I kept are the new ones? It's not as simple as 50/50 -- because I might have disposed of one good contact, but kept the other good one. Or both might be good. Or both might be bad.

Any help?