A cylinder with adiabatic walls is closed at both ends and is divided into two compartments by a frictionless adiabatic piston. Ideal gas is filled in both (left and right) the compartments at same P, V, T. Heating is started from left side until pressure changes to $$27 P/8$$. If initial volume of each compartment was 9 litres then the final volume in right-hand side compartment is __________ litres. (for this ideal gas $$C_P/C_V = 1.5$$)