https://purengom.com/2009/05/30/%ec%97%ac%ed%96%89%ed%95%98%eb%8b%a4-%eb%82%ad%ed%8c%a8-%eb%b3%b8-%ea%b8%b0%ec%96%b5-%ea%b0%80%ec%9d%b4%eb%93%9c%eb%b6%81%ec%9d%98-%ed%8e%98%ec%9d%b4%ec%a7%80-%ed%95%b4%ec%b2%b4/