https://blog.gongquiz.com/2021/07/23/%eb%82%b4%ec%95%95%eb%b0%a9%ed%8f%ad%ea%b5%ac%ec%a1%b0%ec%97%90%ec%84%9c-%ec%95%88%ec%a0%84%ea%b0%84%ea%b7%b9safe-gap%ec%9d%84-%ec%a0%81%ea%b2%8c-%ed%95%98%eb%8a%94-%ec%9d%b4%ec%9c%a0%eb%a1%9c/