input: Write a python function to calculate the area of a rectangle.
  assert Rectangle(5, 3).area() == 15
  assert Rectangle(10, 10).area() == 100
  assert Rectangle(7, 2).area() == 14
output: class Rectangle:
      def __init__(self, width, height):
          self.width = width
          self.height = height
  
      def area(self):
          return self.width * self.height