A disc of mass 'm' and radius R is free to rotate in horizontal plane about a vetical smooth fixed axis passing through is centre. There is a smooth groove along the diameter of the disc and two small balls of mass m/2 each are placed in it on either side of the centre of the disc as shown in fig. The disc is given initial angular velocity `omega_(0)` and released. The angular speed of the disc when the balls reach the end of the disc is -