Help with dropdown menu excel

Hi everyone!

Please see attached screenshot. 

I have created a dropdown menu with various expense posts as illustrated. What I'm trying to figure out is how to make them go into the overview collum on the left side. Example: If I choose Food and enter $100 I want that amount to add to the total on the left side under expenses.. Anyone here know what formula or smartfunction I would need to use for this to be possible?