As the others have said, it was 2 slices. You can account for that in your weekly syns. If you think to pre SW (and this is an assumption based on my own behaviour), 2 slices would never have been enough...I would have scoffed the lot and had fries on the side. People who don't 'diet' have pizza every so often, as I say all the time, it's what you do after it. Don't look at it as a big failure that you can't return from, you can and easily. It's just getting the right mindset which is the hardest part.
Don't beat yourself up..bet it was yummy haha!