How do I insert a DVD into my Mac?

I use an iMac Late 2015 version OS X El Capitan 10.11.1 (I am going to update to Beta 10.11.5) with 4K Built-in Retina. I need help on how to insert a DVD into my Mac. I've looked for it behind, in front, on the top and bottom of my Mac. Please help. I don't want to purchase anything to do this but if I have to, I will. If you tell me to purchase something that could put DVDs into Macs, please check if it's on Amazon and please tell me the name and/or link.