What batteries does the Graco Slim Fit swing require and how do I install them?

The swing requires four "D"-LR20 (1.5V) cell batteries, which are not included. With the baby out of the swing, insert a coin (penny, nickel, or dime) into the coin slot on the battery compartment lid to open it, then insert the four batteries. Make sure the batteries are installed correctly with proper polarity, as the swing will not run if batteries are inserted backwards. (Page 20)