MAGIC SQUARE: Calculate A+B+C

A nun is standing outside a pub and a man comes round the corner, planning to grab an after-work bevy. The nun immediately points at him, and intones:

"Before you enter this den of sin and debauchery, think of your mother and father!"

The man wipes away a tear, and says "They're dead, God bless 'em. They're dead, in heaven."

"Well," says the nun, "Then think of the damage the alcohol will do to your brain!"

"What? What are you talking about?" the man asks. "Have you ever had a drink?" The nun says she has not. "Then how can you talk to me about alcohol? I'll tell you what I'll do," he continues, "I'll buy you a drink, and after you've drunk it, then you can talk to me about alcohol. What'll you have?"

"I don't know," says the nun. "What do ladies usually drink?"

"Gin," he replies.

"Oh, alright," she says. "But - but can you put it in a cup, so nobody notices." The man nods and walks into the bar, calling out to the bartender.

"Bartender! I'll have a beer, and a double gin in a cup!"

"It's that bloody nun outside again, isn't it?"