Creating a discrepancy for schedule

Hello, I am working on creating a user friendly schedule rotating schedule. 

I am trying to see how I can create a formula or a way to indicate when I am missing a "rotation"

For example, in the following image. I am missing "DU" and "C".

Any way I can do this?

The numbers next to the Key is how many should be on the hour. I hope this makes sense. 

Thanks for all your help!