A massive planet of radius R has a diametrical hole as shown in figure, such that hole does not effect the spherecity of the
planet. The planet has uniform density. Two equal mass A and B [m << M] are simultaneously released, from positions shown. If the collisions are elastic then the total distance traveled by mass A after being released at the time of 6^th collision is : [ M$\to$mass of planet, m$\to$ mass of A and B neglect the gravitational interaction between mass A and mass B.]