Numbers.app - How to add up items from multiple sheets and popup lists

Based on the "Personal Budget" template:

I want to use the Personal Budget template and modify it to my needs — I want a total sheet that will add up information from 12 data sheets (for each month). The template example has a total page and 1 data sheet by default.

What would the formula be to get data for a category, across multiple sheets? In my case all 12 data sheets?

For example: A8 is "Travel" SUMIF Jan::Transactions::Category,A8,Jan::Transactions::$Amount)

This gives me the amount of all "travel" expenses for the sheet "Jan" but how would I have the combined total for Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec?

What would the formula look like?

Sorry, not very well versed in spreadsheets. Thanks!

— James