Unfortunately, discounts like this are hard to produce in Rules (the recommended way to do it).

1) What I would recommend is use the commerce_discount module which provides an "order level" discount type.

2) Then I would create a function in a custom module that looks at the cart contents and returns a number for the discount. Possibly another that provides a true/false if the discount should apply.

3) Finally, I would use rules to apply the order level discount based on your custom true/false condition and applies the amount of discount provided by your custom calculation function.