Hi Hectictar,
I have actually done that.
It is a probablility question so showing individual combinations that fit or do not fit the condition is not going to help.
Have you tried to work through mine and/or Alan's answer?
Since I stuffed up a little on my wording, would you like me to do it slowly with loads of diagrams to help you understand? (Send me a message if you do)
If you are not super interested I will not bother
but
if you will put a lot of effort into understanding what I do I will be more than happy to present my answer with much more detail. 