TBH you can't get pregnant from 1 sperm. The zona pellucida which is a thick layer covering the egg is reduced only by a chemical excreted by sperm cells. It takes quite a few to reduce the zona pellucida enough for 1 sperm to penetrate it.
This has to be the best question ever "What are the odds that a girl can get pregnant if precum was on the tip of the penis and was less than 1 inch inside the vagina but was protected by a torn peice of saran wrap?"