Find the length of red arc AB

A blonde, a brunette, and a redhead all work at the same office for a female boss who always goes home early. "Hey girls," says the brunette, "Let's go home early tomorrow. She'll never know." The next day, they all leave right after the boss does. The brunette gets some extra gardening done, the redhead goes to a bar, and the blonde goes home to find her husband having sex with the female boss! She quietly sneaks out of the house and returns at her normal time. "That was fun," says the brunette. "We should do it again sometime." "No way," says the blonde. "I almost got caught!"