Restitution coefficient

Two balls $A$ and $B$ with the same mass are pendent from the ceiling. First, ball $A$ is raised by $90$ ° to the left and then dropped, so that it collides with ball $B.$ Then, ball $B$ goes up by $60$ ° to the right. What is the restitution coefficient between the two balls? (Ignore air resistance and the masses of the strings.)