One calculator is set at 100 and the other at zero.

The player whose calculator is set at 100 may only subtract and the player whose calculator is set at zero may only add. They may not clear their calculators.

The players take turns at either taking away or adding any number they choose on their calculators.

The first to go below the other (for the one who started at 100) or above the other (for the one who started at zero) is the winner.

Exchange starting numbers for the next game. The previous winner goes first.

Decimals may be used!