Formula to Render zero if another field is empty

Hi, 

I have a to calculate hours worked on a spreadsheet but the first hour is $50 and the subsequent hours are $35/HR ..   so I built a column to add $50 which is populated when the hours are entered, and a formula in the next column taking the (total hours, - 1 *35) then add the two columns in the third column. 

It works fine except when there are no hours entered, the formula field is rendering -35 because it's calculating 0-1*35...  how do I get that column to render 0 when there are no hours entered.. 

I need to keep it at zero because I'm summing it for a total amount to be paid out, and the -35 is causing the total to be incorrect. 

I've attached the spreadsheet to make more sense.