Two blocks A and B of same mass $M$ are connected with each other with an ideal string of length $2\mathcal{l}$ passing over an ideal pulley. The block A is connected to a light pan C with an ideal string as shown in figure. A particle of mass $\frac{M}{2}$  is dropped on pan from height $\frac{\mathcal{l}}{2}$  as shown. If collision between particle and pan is perfectly inelastic, acceleration of B just after the collision, is :-