“(-1) x (-1) = +1” 　Why ?

Hi, steemian!

Can you prove “(-1) x (-1) = +1” ? The simple proof is below.

(-1) x 0 = 0

(-1) x (-1+1) = 0

(-1) x (-1) + (-1) x (+1) = 0

(-1) x (-1) + (-1) = 0

(-1) x (-1) = +1