https://blog.gongquiz.com/2022/09/16/%eb%8b%a4%ec%9d%8c-%ec%a4%91-%eb%b3%b4%ec%9d%bc%eb%9f%ac-%ec%9a%a9%ec%88%98%ea%b4%80%eb%a6%ac%ec%97%90%ec%84%9c-%ea%b2%bd%eb%8f%84hardness%ec%99%80-%ea%b4%80%eb%a0%a8%eb%90%98%eb%8a%94-%ed%95%ad/