What are the chances that I might get pregnant?

Hello, I’ve been browsing the internet but unfortunately I cannot find a clear answer, so I’m asking here: an hour ago an accident occurred where my boyfriend’s sperm ended up on my vulva. I’m six days before my period and I’m planning on taking Plan B as I’ll be too nervous if I don’t. I’ve read that it’s possible to get pregnant that way, but considering that my period is expected to come soon, what would you say the chances are?