input: Write a function to implement a Python class named Sum which can calculate the sum of two integers. The class should be initialized with two integers, x and y. It should have a method named add that returns the sum of x and y. If one of the values is not provided or is not an integer, the method should return None.
  
  class Sum:
      def __init__(self, x=None, y=None):
          self.x = x
          self.y = y
          
      def add(self):
          if self.x is None or self.y is None:
              return None
          if not isinstance(self.x, int) or not isinstance(self.y, int):
              return None
          return self.x + self.y
  
  assert Sum(5, 3).add() == 8
  assert Sum(0, 0).add() == 0
  assert Sum(10, -5).add() == 5
  assert Sum(7, None).add() == None
  assert Sum(None, 9).add() == None
  assert Sum("10", 5).add() == None
output: 
  class Sum:
      def __init__(self, x=None, y=None):
          self.x = x
          self.y = y
          
      def add(self):
          if self.x is None or self.y is None:
              return None
          if not isinstance(self.x, int) or not isinstance(self.y, int):
              return None
          return self.x + self.y
  