What is the Devil's arithmetic?

She learned all the tricks to avoid being chosen and knew that if she followed them the odds were, she wouldn’t be taken this time. Part of her revolts ahain the insanity of the rules and part is grateful because in a world of chaos any guidelines help. “Each day she remains alive, she remains alive. One plus one plus one. The Devil’s arithmetic, Gitl calls it.” (pg. 135) And so one day erodes into the other and Hannah’s memories become camp memories only. One day, Gitl organizes a jump rope for the children to play. Later, that night Masha from Karkow hangs herself with it when she learns her husband and son had gone up the smokestack. It is the way of the camp.

The specific meaning for the Devil's Arithmetic is that it refers to the the idea that for every one person who dies instead of you is one more day that you life and one less chance you will be Chosen for the gas chamber. It is an addition problem designed by evil or the devil.