https://purengom.com/2010/10/21/%ec%95%84%ec%9d%b4%ed%8f%b0-4-%ec%84%9c%eb%b9%84%ec%8a%a4%eb%a5%bc-%eb%b0%9b%ec%95%84%eb%b3%b4%eb%a9%b4%ec%84%9c-%eb%8a%90%eb%82%80-%ec%a0%90-%ec%9a%94%ec%a7%80%eb%b6%80%eb%8f%99-%ec%95%a0%ed%94%8c/