How to average multiple images into one with ImageMagick

convert -average darky1.jpg darky2.jpg lighty1.jpg lighty2.jpg l.ppm
convert -average lighty1.jpg lighty2.jpg lighty1.jpg lighty2.jpg d.ppm

l.ppm is much lighter than d.ppm. The order of images matters (but should not).

The same for using evaluate-sequence mean.

How to do it properly?