What is the missing number?

The new associate pastor, nervous about hearing confessions asks an older priest to listen in. Several penitents later, his mentor offers a few suggestions.“Cross your arms over your chest and rub your chin with one hand,” he says. “Try saying things like, ‘I see, yes, go on. I understand. How did you feel about that?”The new priest tries out the words and gestures. The old priest says, “Good, now, don’t you think that's a little better than slapping your knew and saying, ‘No way! You did what?'"