Counting Formula

I've got two columns one recording hours worked on a project in a week and the other giving a running total. E.g column A is hours recorded and Column B is total hours. so in A1 I put 4 and in A2 5. A simple sum formula adds both together to give 9 in cell B2. However cells b3 to b10 also have a figure of 9. Can I in cell b3 say if cell a3 is greater than 0 add it to cell b2?

Thanks

=if(a2="","",sum($a$1:a1))