Auto-fill collumn with SUM function

Hello,

Im making a sheet in Google Drive, but I figure the functions won't be too different from Excel so I post my question here.

I've been struggling with this for a few weeks now.
I have two columns that get automatic updates, and there's one collumn which should display the total of those two.
Is there a way that this colum gets automatically updated as well ?

When I use the SORT function, the cells in the colum get automatically update by the Continue function.
Somehow I don't get this to work with the SUM function.

Any help would be appricated

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