https://blog.faceutil.com/2017/03/27/%ec%96%b4%eb%b3%b4%ec%9d%b4%eb%93%9c-%ec%97%94-%ec%84%9c%eb%b0%94%ec%9d%b4%eb%b8%8c-1-5-avoid-n-survive-1-5-%d0%b3/