How can only count certain cells in excel 2013.?

So I m trying to keep track of my overtime hours per week. In column A is have my goal hours, and in column B is the hours actually worked. So I have column A filled out and column B will be filled in as the weeks go on, and the total amounts of each column will be added up in a cell at the bottom of the list. Adding column B is easy. What I need help doing is counting column A. I want to only count the cells corresponding to weeks that have already been worked. So say I have A1-A10 all with a value of 8 goal hours for those 10 weeks. And I ve only worked 3 weeks B1-B3 and I worked 9 hours each of those weeks. So I want B11 to say 27 and A11 to say 24. Is there a way I can have the cells in column A only counted as the cell in Column B is entered.