input: Write a function to complete the given Python class named Sum that can calculate the sum of two integers. The class should have an `add` method that returns the sum of the two integer attributes `x` and `y`. If the given class is empty, then return the completed class with the `add` method.
  
  
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          return self.x + self.y
  
  # Testing the completed class
  obj1 = Sum(5, 3)
  assert obj1.add() == 8
  
  obj2 = Sum(-1, 1)
  assert obj2.add() == 0
  
  obj3 = Sum(0, 0)
  assert obj3.add() == 0
  
  obj4 = Sum(100, -100)
  assert obj4.add() == 0
  
  obj5 = Sum(7, 14)
  assert obj5.add() == 21
  
output: def __init__(self, x, y):
      self.x = x
      self.y = y
      
  def add(self):
      return self.x + self.y