Find the length of red arc AB

A doctor wanted to get off work and go fishing, so he approached his assistant. "Murphy, I am going fishing tomorrow and don't want to close the clinic. I want you to take care of the clinic and take care of all me patients."
"Yes, sir!" answers Murphy.
The doctor goes fishing and returns the following day and asks: "So, Murphy, how was your day?"
Murphy told him that he took care of three patients... "The first one had a headache so he did...So I gave him Paracetamol."
"Very good, Murphy lad, and the second one?" asks the doctor.
"The second one had indigestion and I gave him Gaviscon," says Murphy.
"Great! You're good at this and what about the third one?" asks the doctor.
"Sir, I was sitting here and suddenly the door flies open and a young gorgeous woman bursts in. Like a bolt outta the blue, she tears off her clothes, taking off everyting including her bra and her panties and lies down on the table, spreading her legs and shouts: 'HELP ME! For five years I have not seen any man!'"
"Oh my... What did you do?" asks the doctor.
"I put drops in her eyes."