Conditional Sum and Loop

[rlsublime]

I am trying to insert a sum in C3 onwards only if there is a value in D3 onwards. The sum should be A3+B3 onwards. The range is dynamic which is why i want the code to look at D column, see if there is a value there and then sum if there is a value in D3 onwards. I think some sort of For Loop would accomplish this but not sure how to structure it. Thanks for the help! I have attached the sample spreadsheet. I am trying to populate the C Column