Arggg!! How do I subtract multiple numbers in a column?

Can anyone tell me how to subtract multiple numbers down a column in Numbers? I want to start with my figure in A2 and I want to be able to subtract the figures in cells A3 down to A19 from it, with the answer showing in A20. I'm sure this is very basic, but can't seem to make it work whatever I try 😢

Obviously I am a novice (to both Mac's and Spreadsheets) 😊

Many thanks.