You are making a diffraction grating that is required to separate the two spectral lines in the sodium D doublet, at wavelengths 588.9950 and 589.5924 nm, by at least 1.2 mm on a screen that is 65 cm from the grating. The lines are to be ruled over a distance of 1.2 cm on the grating. What is the minimum number of lines you should have on the grating?