Thirteen!

A young man was strolling down a street behind an assisted living center. As he passed the large building with a fence around it, he heard a group of people chanting "Thirteen, thirteen, thirteen, thirteen" over and over again.

Curious, he tried to see over the fence, but couldn't. Then he spotted a hole in one of the boards and leaned over to take a look.

He had just managed to see some old people sitting in deckchairs chanting, when a finger came out of nowhere and poked him in the eye.

As he staggered back, the old people started chanting, "Fourteen, fourteen, fourteen, fourteen..."