Condom Broke, Plan B

hey guys. So I had sex using a condom and all went fine until he went and took it off and realised it ripped during sex. I immediately started panicking considering I'm not trying and not wanting to conceive. I went to the store and got a plan b and took it less than 2 hours after. what's the percentage it will work. I also am supposed to start my period is 3 days.