Find a missing number

A guy decides to take off work early from work and go drinking. He stays in the bar until it closes at 2 a.m. By then, he is extremely drunk.

When he gets back to his house, he doesn't want to wake anyone up, so he takes off his shoes and starts tiptoeing up the stairs.

Halfway up the stairs, he loses his balance, falls over backwards, and lands flat on his rear end.

That wouldn't have been so bad, except that he had a couple of empty pint bottles in his back pockets and they broke, carving up his rear end terribly. But he was so drunk he didn't know he was hurt.

A few minutes later, as he was undressing, he saw some blood. He checked himself out in the mirror and, sure enough, his rear end is cut up something terrible. He repaired the damage as best he could under the circumstances and went to bed.

The next morning, his head was hurting and his rear was hurting, and he was lying under the covers trying to think up a good story, when his wife came into the bedroom.

"Well, you really tied one on last night," she said. "Where'd you go?"

"I worked late," he said, "and I stopped off for a couple of beers."

"A couple of beers? That's a laugh!" she replied. "You were plastered last night, and you know it! Where'd you go?"

"What makes you so sure I got drunk last night, anyway?"

"Well," she replied, "my first big clue was when I got up this morning and found a bunch of band-aids stuck to the mirror."