I thought it was a week afterward, not two. Regardless, from what I've read that method, which is often referred to the Catholic method of birth control, isn't to be used liberally as girls can often miscount their days or even start counting from a day where they can still get pregnant. Still, I'm better off paying $8 for a pack of rubber sleeves than potentially paying thousands of dollars for a crotch-monster I didn't want to happen...