Find the length of red arc AB

A highly timid little man, ventured into a biker bar in the Bronx and clearing his throat asked, "Um, err, which of you gentlemen owns the Doberman tied outside to the parking meter?"
A giant of a man, wearing biker leathers, his body hair growing out through the seams, turned slowly on his stool, looked down at the quivering little man and said, "It's my dog. Why?"
"Well," squeaked the little man, obviously very nervous, "I believe my dog just killed it, sir."
"What?" roared the big man in disbelief. "What in the hell kind of dog do you have?"
"Sir," answered the little man, "it's a little four week old female puppy."
"Bull!" roared the biker, "how could your puppy kill my Doberman?"
"It appears that your dog choked on her, sir."