What are the possibility of getting pregnant?

What are the possibility of getting pregnant with just humping naked? His penis was literally on my vagina's lips and slid inside two times for like one or two centimeters. He did not ejaculate and it was without any kind of protection (I got off the pill two months ago and starting my period tomorrow, this happened yesterday).

I know it is very unlikely but since I know precum can get you pregnant I don't want any chance yet (in fact I'm getting again on the pill asap)