Calculating OT in Excel

Hello,

I am making a spreadsheet to track hrs worked up to 40 at one rate and anything over 40 at a 1.5 rate. I am using the formula  =sum(n6:n12) to total my regular hours. I display this value in N15. I would like it to just calculate the first 40hrs and stop. I want anything over 40hrs to be displayed in N17 and will calculate that using a rate of 1.5. 

Thanks for any help given.