Two blocks of masses `M_(1)=4` kg and `M_(2)=6kg` are connected by a string of negligible mass

Two blocks of masses `M_(1)=4` kg and `M_(2)=6kg` are connected by a string of negligible mass passing over a frictionless pulley as shown in the figure below. The coefficient of friction between the block `M_(1)` and the horizontal surface is 0.4. when the system is released, the masses `M_(1)` and `M_(2)` start accelrating. What additional mass m should be placed over `M_(1)` so that the masses `(M_(1)+m)` slide with a uniform speed?