input: Write a function to complete the Python class Sum so that it can calculate the sum of two integers. The class should have an instance method called `add` that returns the sum of the two integers initialized in the class constructor.
  
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          return self.x + self.y
  
  assert Sum(2, 3).add() == 5
  assert Sum(-1, 1).add() == 0
  assert Sum(10, -5).add() == 5
  assert Sum(0, 0).add() == 0
  assert Sum(-3, -3).add() == -6
output: def __init__(self, x, y):
      self.x = x
      self.y = y
          
  def add(self):
      return self.x + self.y