input: Write a function to complete the Python class Sum so that it can calculate the sum of two integers. The class should include an `add` method that returns the sum of `x` and `y`.
  
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          return self.x + self.y
  
  assert Sum(3, 5).add() == 8
  assert Sum(10, -2).add() == 8
  assert Sum(0, 0).add() == 0
  assert Sum(-5, -3).add() == -8
  assert Sum(100, 200).add() == 300
output: 
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          return self.x + self.y
  