Help with Macro: summing together/roll over rows (Energy Trading, see example)

(I attached the file so it will make more sense.) Basically what I need is a macro that by clicking it will sum cell C12 and F12 placing the sum in F12, C13 and F13 placing the sum in F13, and so on. I need it to do this on a row by row basis. Each row basically represents a different product. Also, I would like a message to pop up when the macro is hit to ask "are you sure" Then by clicking yes or no it will do so accordingly. I've tried programming it in VBA using =sum with no luck. I would greatly appreciate any and all help. Thanks!