Windows 8 New CD (Dell Inspiron 3521) Need Product Key

Hi Friends, 

I have purchased DELL Inspiron 3521 which comes with Windows 8 CD. Initially I installed Windows 7 but now would like to install Windows 8. This CD has never been installed on my laptop or any other machine as it is brand new and sealed one. 

When I boot the computer with DVD ROM, it asked for Product Key. I didn't find Key on CD Cover. Then I logged on Computer and open the CD folder to see of there is any file which could help me to find CD Key but no luck. 

Can someone guide me how can I get the Product Key of this CD? 

Regards, 

i