control iMac in one room to my Apple TV in another room

My 2017 27" 5K iMac is in the computer room. My AppleTV3, connected to my 65" 4K Samsung TV, is in the living room, about 40 feet away. Displaying my iMac to the AppleTV works beautifully. But my Apple wireless keyboard and mouse, which work fine when next to the iMac, won't function from the Living Room. How can I extend that (or connect something else) so I can control the iMac from the Living room?