Two equal blocks, each of mass $$M$$, hang on either side of a frictionless light pulley with a light string. A rider of mass $$m$$ is placed on one of the blocks as shown. When the system is released, the block with rider descends a distance $$H$$ till the rider is caught by a ring that allows the block to pass through. The system moves a further distance $$D$$ taking time $$t$$. In such a situation, the acceleration due to gravity is