https://blog.faceutil.com/2017/04/27/56-%ed%8c%8c%ec%9b%8c%ed%8f%ac%ec%9d%b8%ed%8a%b8-%ec%97%91%ec%85%80-%eb%a7%81%ed%81%ac-ppt-%ec%9b%8c%ed%81%ac%ec%8b%9c%ed%8a%b8-%ec%82%bd%ec%9e%85-%d1%8a/