https://blog.gongquiz.com/2021/04/22/%eb%ac%b8%ec%9d%b4-%ec%9e%a0%ea%b2%bc%ec%9d%84-%eb%95%8c-%ed%95%b4%ea%b2%b0%eb%b0%a9%eb%b2%95-the-solution-when-the-door-is-locked-%e3%82%af/