https://blog.gongquiz.com/2022/05/01/%eb%8b%b9%ea%b5%ac-billiard-%ea%b0%95%ec%83%81%ea%b5%ac%ec%9d%98-%ec%83%81%ea%b5%ac%eb%8b%b9%ea%b5%ac-75%ed%8e%b8-%eb%82%9c%ea%b5%ac%ed%92%80%ec%9d%b4-%ec%93%b0%eb%a6%ac%eb%b1%85%ed%81%ac-1-2/