The Chanting

I was walking by the local mental institution when I noticed I could hear the patients outside chanting.

"13...13...13...13..."

There was a ten foot tall wooden fence around the property, so I couldn't see what was happening. I had almost completely walked by the entire institute when I noticed a pretty sizeable hole near the ground at the corner of the fence. I decided to stoop down and take a peek at what was going on.

BAM!!!

Suddenly, something jabbed me in the eye hard enough to knock me on my ass! While I was still processing what had happened, I noticed the chanting had changed.

"14...14...14..."