Let $ y = x $ be the equation of a chord of the circle $ C_1 $ (in the closed half-plane $ x \ge 0 $) of diameter 10 passing through the origin. Let $ C_2 $ be another circle described on the given chord as its diameter. If the equation of the chord of the circle $ C_2 $, which passes through the point $ (2, 3) $ and is farthest from the center of $ C_2 $, is $ x + ay + b = 0 $, then $ a - b $ is equal to