A block of mass M=4 kg is kept on a smooth horizontal plane. A bar of mass m=1 kg is kept on it. They are connected to a spring as shown and the spring is compressed. Then, what is the maximum compression in the spring for which the bar will not slip on the block when released, if coefficient of friction between them is 0.2 and spring constant =1000 N/m (Take g=10 m/s2)