Calculate pressure of PCl5 after system has reached equilibrium?

At 250degrees celsius, the equilibrium constant kp for the reaction PCl5(g) <--> PCl3(g) + Cl2(g) is 1.8. Sufficient PCl5 is put into a reaction vessel to give an initial pressure of 4.00 atm at 250degrees celsius. Calculate the pressure of PCl5 after the system has reached equilibrium.