input: Write a function to complete the Python class Sum below so that it can calculate the sum of two integers. The class should have an __init__ method to initialize two attributes x and y, and an add method that returns the sum of these two attributes. Use the following assert statements to test the implementation.
  
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          return self.x + self.y
  
  assert Sum(5, 3).add() == 8
  assert Sum(-2, 2).add() == 0
  assert Sum(0, 0).add() == 0
  assert Sum(-5, -3).add() == -8
  assert Sum(10, -4).add() == 6
output: 
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          return self.x + self.y
  