Calculate the number 1391

Sam has been in business for 25 years and is finally sick of the stress. He quits his job and buys 50 acres in Alaska as far from humanity as possible. He sees the postman once a week and gets groceries once a month. Otherwise it's total peace and quiet. After six months or so of almost total isolation, someone knocks on his door. He opens it and there's a big, bearded man standing there.
"Name's Lars ...Your neighbor from forty miles away....Having a birthday party Friday ... Thought you might like to come. About 5...
"Great," says Sam, "after six months out here I'm ready to meet some local folks. Thank you."
As Lars is leaving, he stops. "Gotta warn you...There's gonna be some drinkin'."
"Not a problem... after 25 years in business, I can drink with the best of em."
Again, as he starts to leave, Lars stops. "More'n'likely gonna be some fightin' too."
Sam says, "Well, I get along with people. I'll be there. Thanks again."
Once again Lars turns from the door. "I've seen some wild sex at these parties, too."
"Now that's really not a problem," says Sam. "I've been all alone for six months! I'll definitely be there. By the way, what should I bring?"
Lars stops in the door again and says, "Whatever you want, just gonna be the two of us".