Calculate the number 8638

An elderly man calls his son who lives in another city and says: "I hate to ruin your day, but I have to tell you that your mother and I are divorcing, forty-five years of misery is enough".

"Dad, what are you talking about?"

"We can't stand the sight of each other any longer," the old man says. "We're sick of each other, and I'm sick of talking about this, so you call your sister and tell her."

Frantic, the son calls his sister, who explodes on the phone. "No way they're getting divorced", she shouts. "I'll take care of this."

She calls her parents immediately, and says to her father: "You are not getting divorced. Don't do a single thing until I get there. I'm calling my brother back, and we'll both be there tomorrow. Until then, don't do a thing, do you hear me?!"

The old man hangs up his phone and turns to his wife. "Okay," he says. "They're coming for our anniversary and paying their own way. Now what do we tell them for your birthday?"