input: Write a function to complete the Python class so that it can calculate the sum of two integers. If the given class is already complete, return True, else return False.
  assert Sum(3, 4).add() == 7
  assert Sum(5, -2).add() == 3
  assert Sum(0, 0).add() == 0
  assert Sum(10, -10).add() == 0
  assert Sum(-5, -5).add() == -10
output: def add(self):
      return self.a + self.b