Shape类派生了Circle类和Rectangle类，请阅读主函数代码，完成Shape类、Circle类、Rectangle类的定义

#include <iostream>
#include <cmath>
using namespace std;

class Shape {
public:
    virtual double Area() const = 0;
};

class Circle : public Shape {
private:
    double radius;
public:
    Circle(double r) : radius(r) {}
    double Area() const {
        return M_PI * radius * radius;
    }
};

class Rectangle : public Shape {
private:
    double width;
    double height;
public:
    Rectangle(double w, double h) : width(w), height(h) {}
    double Area() const {
        return width * height;
    }
};

int main() {
    Shape* s1 = new Circle(5);
    Shape* s2 = new Rectangle(4, 6);
    cout << "Circle area: " << s1->Area() << endl;
    cout << "Rectangle area: " << s2->Area() << endl;
    delete s1;
    delete s2;
    return 0;
}