type Shape interface {
	area() float64
}
 
type Rectangle struct{
	height float64
	width float64
}
 
type Circle struct{
	radius float64
}
 
func (r Rectangle) area() float64 {
	return r.height * r.width
}
 
func (c Circle) area() float64 {
	return math.Pi * math.Pow(c.radius, 2)
}