A Cherenkov counter containing nitrogen gas at pressure is located on a charged particle beam with

A Cherenkov counter containing nitrogen gas at pressure Π is located on a charged particle beam with momentum p = 20 GeV. The dependence of the refraction index on the pressure Π is given by the law n – 1= 3 x 10^-9Π (Pa). The Cherenkov detector must see the π and not the K. In which range must the pressure be?