What Are The Chances Of Getting Pregnant Through Non Penetrative Sex?

I was intimate with my gf. We both are virgins. I guess i dropped a drop of semen on her vagina externally. This happened on 14th jan. After 40hrs my gf took an unwanted72. On 16th jan she had her periods but the flow was less. now this feb, her due date of her periods was 16th feb but she did not have her periods till today 23rd feb. She took unwanted 72 for the first time. Pregnancy test also indicated negative. please advice us what to do?