Random Event on "any Button.Click"

I am trying to do one of two things. I need ads to show when buttons are clicked, but I do not want to show an ad every time a button is clicked because I do not want to annoy the users.
My two ideas on how to implement "occasional" instances of the interstitial ads, are; to simply make the ads appear on every other click of a button, or; to make the ads appear by some kind of randomization and thus, be able to display ads somewhat more than with the "every other click" method but still not on every click... again, so as not to annoy users.

The blocks shown in the picture are what I am currently working with.

Any help or suggestions are enormously appriciated!

Perhaps like this:


You can do more mathematics to reduce/increase frequency of random success