input: Write a function that completes the Python class `Sum` so that it can calculate the sum of two integers. The `Sum` class should have an `__init__` method to initialize the two integers and an `add` method to compute and return their sum.
  
  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(0, 0).add() == 0
  assert Sum(-1, 1).add() == 0
  assert Sum(10, -5).add() == 5
  assert Sum(-3, -3).add() == -6
output: 
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          return self.x + self.y
  