A vertical closed cylinder of cross-sectionaal area A is divided into two equal parts by a heavy insulating movable piston of mass mp. The top part contains nitrogen at a temperature T1 and pressure P1, and the bottom part is filled with oxygen at a temperature 2T1. The cylinder is turned upside-down. To keep the piston in the middle, the oxygen must be cooled to T2 = T1/3, with the temperature of the nitrogen remaining at T1. Find the initial pressure of oxygen Pi.