Numbers formula help

Hi, I'm having trouble creating a formula in Numbers and would appreciate any help.

I'm creating a spreadsheet to track employee annual leave, and most columns are set up correctly, but I can't work out how to create a column to track how many days have been taken so far.

In the attached screen-grab, I want column D to keep a sum total of the number of days taken by adding the entries so far from column C. I've filled in the first 4 entries using a formula for each cell but I want to be able to spread the 'rule' down the column so that other cells auto-update.

At this rate it'd be quicker for me to do it manually but I know I must be missing something really simple!

Thanks,
Emily
(I'm very familiar with Macs and computing generally, but unfamiliar with Numbers spreadsheets...)

 

Fixed

Thanks quickie14, I'm embarrassed I didn't think of doing it that way!
All sorted.