input: Write a function to implement the `add` method in the `Sum` class so that it returns the sum of two integers stored in the class instance variables. If the class is instantiated with integers, the method should return the sum of those integers. If the class is instantiated with non-integer values, the method should raise a `ValueError`.
  
  
  assert Sum(3, 4).add() == 7
  assert Sum(-1, 1).add() == 0
  assert Sum(0, 0).add() == 0
  assert Sum(10, -10).add() == 0
  
output: def add(self):
      if isinstance(self.a, int) and isinstance(self.b, int):
          return self.a + self.b
      else:
          raise ValueError