https://itmanual.net/%ec%97%91%ec%85%80-if-%ed%95%a8%ec%88%98-%ec%82%ac%ec%9a%a9%eb%b2%95-%eb%85%bc%eb%a6%ac%ed%95%a8%ec%88%98-and-or-not-%ec%a1%b0%ed%95%a9/