How many calories in watermelon?

I'm trying to count my calories really carefully, and I want to know many many calories there are in one slice of watermelon. It was a sugar baby watermelon i think, the kind you buy three at a time that are much smaller than normal watermelon, and it was about 1 1/2 inches thick, three inches long at the bottom, and two inches high and cut in a triangular shape. Can you give me an approximate amount of calories in that? A calorie website said about 86 for 1/16 of a normal watermelon, but I think that's too low. I mean, doesn't watermelon have tons of sugar in it?