Find the length of red arc AB

I said to my father: "Dad, I want to get married."

He said: "Alright son, who do you want to marry?"

I said: "I'd like to marry Miss Green".

He said: "You can't".

I said: "Why not?"

He said: "She's your half-sister. When I was a lad I had a bike and I got around a bit."

I said: "Alright, I'll marry Miss White."

He said: "You can't, she's your half-sister. Forget about it."

Well, I was a bit despondent and I walked around and my mum said to me: "What's wrong with you?"

I said: "Well, I said to Dad I wanted to marry Miss Green and he said I couldn't because she's my half-sister. I said, "All right, I'll marry Miss White."

He said: 'You can't, she's your half-sister."

She said: "Look, you go and marry which one you like. He's not your father anyway!"

Max Miller (1894-1963)

Picture: REX Features