https://soooprmx.com/python101-003-%ec%9e%85%eb%a0%a5-%ec%82%ac%ec%9a%a9%ec%9e%90%eb%a1%9c%eb%b6%80%ed%84%b0-%ec%9e%85%eb%a0%a5%eb%b0%9b%ea%b8%b0/