I had windows 8.1 but want to go back to windows 8 with clean install"the product key entered does not match any of the windows image available"

I had windows 8.1 and wanted to revert to windows 8 so i by mistake formatted the c:/ drive containing os and now when i try to install windows 8 i get the error "The product key entered..."

And i dont even have recovery image in my pc. I am using a dell laptop and i have windows 8 dvd given by dell which reads" windows 8 recovery media for windows product"

Please guide me asap.