定义一个基类MyGraph，要求有一个纯虚函数double Area()=0。从基类派生一个圆类Circle和矩形类Rectangle，再由Rectangle类派生出Square类，分别给出Area()实现。

重载“>>”，实现对图形信息的输入。在主函数中计算三种形状的面积，并显示输出。 -Define a base class MyGraph, requires a pure virtual function double Area () = 0. Derive a circle and a rectangle class Rectangle Circle class from the base class, and then by Square Rectangle class derived class, are given Area () implementation. Reload " > > "　to achieve the input of graphical information. Calculate the shape of the three main functions in the area, and display output.