Excel Drop Down List Sum Up

I don’t know how to do the following:

I created a drop down list using validation function. the list is about categories of expenses (ie food, petrol etc). I have another column with amount in the same row.
Now at the end of the sheet I want to have a ‘total’ cell, where it sums up all the amount from a given category (so all food expenses are sum up in one cell).

I would really appreciate your help!