Summary

$$ (-1) \times (-1) = (-1) \times (-1) + 0 $$
$$ (-1) \times (-1) = (-1) \times (-1) + (-1) + 1 $$
$$ (-1) \times (-1) = (-1) \times (-1) + 1 \times (-1) + 1 $$
$$ (-1) \times (-1) = ((-1) + 1 ) + 1 $$
$$ (-1) \times (-1) = 0 $$

Reference

  羊堂本舗 - マイナスの掛け算
  http://sheepman.parfait.ne.jp/wiki/%A5%DE%A5%A4%A5%CA%A5%B9%A4%CE%B3%DD%A4%B1%BB%BB

via

  www.textfile.org - 2005-01-12
  http://www.hyuki.com/tf/200501#i20050112202333