Find the length of red arc AB

A professor of chemistry wanted to teach his 5th grade class a lesson about the evils of liquor, so he produced an experiment that involved a glass of water, a glass of whiskey, and two worms. "Now, class. Observe closely the worms," said the professor putting a worm first into the water. The worm in the water writhed about, happy as a worm in water could be. The second worm, he put into the whiskey. It writhed painfully, and quickly sank to the bottom, dead as a doornail. "Now, what lesson can we derive from this experiment?" the professor asked.
Johnny, who naturally sits in back, raised his hand and wisely, responded, "Drink whiskey and you won't get worms."