input: Write a function to create a Python class called Rectangle to represent a two-dimensional rectangle with a length and width, and include a method to calculate the area of the rectangle. If the length or width is negative, return -1 for the area calculation.
  class Rectangle:
      def __init__(self, length, width):
          self.length = length
          self.width = width
  
      def area(self):
          if self.length < 0 or self.width < 0:
              return -1
          return self.length * self.width
  
  assert Rectangle(5, 10).area() == 50
  assert Rectangle(3, 4).area() == 12
  assert Rectangle(-2, 3).area() == -1
  assert Rectangle(0, 0).area() == 0
output: 
  class Rectangle:
      def __init__(self, length, width):
          self.length = length
          self.width = width
  
      def area(self):
          if self.length < 0 or self.width < 0:
              return -1
          return self.length * self.width
  