https://kyungw00k.github.io/2016/04/08/python-%ED%8C%8C%EC%9D%BC-%EC%83%81%EB%8B%A8%EC%97%90-%EC%BD%94%EB%93%9C-%EB%82%B4-%EC%9D%B8%EC%BD%94%EB%94%A9%EC%9D%84-%EB%AA%85%EC%8B%9C%EC%A0%81%EC%9C%BC%EB%A1%9C-%EC%B6%94%EA%B0%80%ED%95%A0-%EA%B2%83/