On number of collisions of billiard balls.On number of collisions of billiard balls.On number of collisions of billiard balls.On number of collisions of billiard balls.On number of collisions of billiard balls.On number of collisions of billiard balls.On number of collisions of billiard balls.On number of collisions of billiard balls.On number of collisions of billiard balls.On number of collisions of billiard balls.