A small block of mass $M$ moves on a frictionless surface of an inclined plane, as shown in figure. The angle of the incline suddenly changes from $60^{\circ}$ to $30^{\circ}$ at point B . The block is initially at rest at A . Assume that collisions between the block and the incline are totally inelastic $\left(\mathrm{g}=10 \mathrm{~m} / \mathrm{s}^2\right)$. The speed of the block at point C, immediately before it leaves the second incline is