input: Write a function named `Sum` that initializes two integer variables `x` and `y`. The function should include a method `add()` that calculates and returns the sum of these two integers. If either of the inputs is not an integer, the method should return `None`.
  
  
  assert Sum(3, 4).add() == 7
  assert Sum(-1, 5).add() == 4
  assert Sum(10, -10).add() == 0
  assert Sum(0, 0).add() == 0
  assert Sum(1.5, 2).add() == None
  assert Sum("3", 4).add() == None
  
output: class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
  
      def add(self):
          if isinstance(self.x, int) and isinstance(self.y, int):
              return self.x + self.y
          else:
              return None