What is the missing number?

A married man and his secretary were having a torrid affair. One afternoon they couldn't contain their passion, so they rushed over to her place where they spent the afternoon making passionate love. When they were finished, they fell asleep, not waking until 8 o'clock that night.

They got dressed quickly. Then the man asked his secretary to take his shoes outside and rub them on the lawn. Bewildered, she did as he asked, thinking him pretty weird.

The man finally got home and his wife met him at the door. Upset, she asked where he'd been. The man replied, "I can not tell a lie. My secretary and I are having an affair. Today we left work early, went to her place, spent the afternoon making love, and then fell asleep. That's why I'm late."

The wife looked at him, took notice of his shoes, and yelled, "I can see those are grass stains on your shoes. YOU LIAR! You've been playing golf again, haven't you?"