Precum

Ok so I'm 22 and on Sunday my boyfriend and I had vaginal (and then anal) sex. Normally he always puts a condom on halfway through so he can finish inside me. Before now I was never worried but someone said you can get pregnant off precum. What's the chances of me getting pregnant off only precum just so I know if I need to buy the plan b pill my I was according to this my chance of pregnancy was 15%