With on one hand

“How are you doing?” said a young guy bumping into his friend at the bar.
“I was fine…until last night.”
“Why? What happened?”
“My girlfriend and I were talking about how many people we had slept with.”
“Oh, what did she say?”
“She said she could count the number of guys she’s slept with on one hand.”
“That’s good, surely?”
“Yeah, I was relieved…but then I saw she was holding a calculator.”

Only Time I Use My Head

Get to work!