Seventeen stairs

There are seventeen stairs in my house. I count them every night on the way up to bed. I've done it since I was a child. It's a habit, nothing more.

Tonight there were eighteen.

I went back down and counted again. Seventeen.

I went up, slowly. Eighteen. The extra one is somewhere in the middle. I can't tell which. They all look the same.

I've been sitting on the landing for an hour. I don't want to go down again, because each time I count, the number going down is seventeen, and the number going up is one more.

The last time it was twenty-one.

And I can hear something coming up them now, counting under its breath.