input: Write a function to complete the Sum class so that it can calculate the sum of two integers. The function should add two integers stored in the instance variables x and y and return the sum. If the instance variables are not integers, the function should raise a TypeError.
  assert Sum(3, 4).add()==7
  assert Sum(-1, -1).add()==-2
  assert Sum(0, 0).add()==0
  try:
      Sum(1, "2").add()
  except TypeError:
      pass
  else:
      assert False, "Expected TypeError"
  assert Sum(100, -100).add()==0
output: def add(self):
      if not isinstance(self.x, int) or not isinstance(self.y, int):
          raise TypeError
      return self.x + self.y