input: Write a function to complete the Python class below so that it can calculate the sum of two integers. The class should have a method named `add` that returns the sum of the integers. If the class is initialized with two integers `x` and `y`, the `add` method should return `x + 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, 4).add() == 7
  assert Sum(-1, 1).add() == 0
  assert Sum(5, -5).add() == 0
  assert Sum(0, 0).add() == 0
  assert Sum(10, 20).add() == 30
output: 
  def __init__(self, x, y):
      self.x = x
      self.y = y
          
  def add(self):
      return self.x + self.y
  