Calculate the number 9181

There was once a blonde woman on a plane to Detroit. She was in the economy class, but after takeoff, she saw an empty seat in first class and moved there. An attendant saw her and said, "Excuse me, ma'am, but you have a ticket for economy class, not first. You cannot stay here." The blonde replied, "I can and I will." The attendant told the copilot, who came and talked to the woman. "Ma'am, we really can't have you staying in this seat, your ticket was for economy." "You can't make me move." The copilot told the captain, who tried to talk her out of the seat but it didn't work. Finally, a man who had heard what had been going on told the attendant to let him have a go at getting the woman out of the seat because he was married to a blonde too, so he knew how to deal with her. After a quick chat with her, she moved. The shocked attendant asked him how he did it. The man replied, "I told her first class wasn't going to Detroit."