Editing Field Values That Get Summed Up - Resets Total Field

I have several fields that I fill in on a form and they all need to add up in the last field. The first way that I tried this worked good for the initial data entry on the form. However if I need to edit one of the fields in the form then it resets the TotalCost field and I have to enter all of the numbers again. Is there some way that I can make this work? Or is this an Access quirk I will have to live with?

I tried to do this two different ways.

Here is the code for the first method:

Option Compare Database
Dim C1 As Long 'For Total Cost
Dim C2 As Long 'For Total Cost
Dim C3 As Long 'For Total Cost
Dim C4 As Long 'For Total Cost

[Code] .....

This way works great the first time that I tried it, I had to re-enter info in all the fields if I wanted to change one.

Here is my second method:

Which didn't work, i received an error message:

"The expression After Update you entered as the event property setting produced the following error: Invalid outside procedure."

And then nothing changes.

Code:
Option Compare Database
Dim C1 As Long 'For Total Cost
Dim C2 As Long 'For Total Cost
Dim C3 As Long 'For Total Cost
Dim C4 As Long 'For Total Cost

[Code] ....

So I know this second method is incorrect.