Two equal masses (1 and 2) are connected by the rope across a pulley and placed on the larger block "A" as shown on the picture. Find the minimal acceleration a of the block "A" along the horizontal so that masses 1 and 2 remain stationary with respect to the block "A". Coefficient of kinetic friction between each of the masses and block "A" is "u", the rope is massless and non-stretchable.