input: Write a python function to calculate the area of a rectangle using a class.
  assert Rectangle(5, 10).area() == 50
  assert Rectangle(3, 7).area() == 21
  assert Rectangle(4, 9).area() == 36
output: class Rectangle:
      def __init__(self, width, height):
          self.width = width
          self.height = height
  
      def area(self):
          return self.width * self.height