Cart discount for individual category

I am hoping someone can help me with this, I am trying to set up a rule but I am having no luck…

We supply 2 different types for pet food, Dray and Frozen. When we ship dry good only in an order, the shipping cost is a lot cheaper than frozen (as we do not need to use ice packs and temperature controlled boxes).

I want to set up a rule where it gives a discount to orders containing only dry goods, we have a category for this.

My problem is that I can’t seem to find a way of excluding other categories, so if a customer orders a mixed basket of dry and frozen the discount for dry is still applied

The page I need help with: [log in to see the link]