input: Write a function that completes the Python class Sum so that it can calculate the sum of two integers. The function should include an add method that adds the two integer attributes of the class and returns the result.
  
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          return self.x + self.y
  
  assert Sum(3, 5).add() == 8
  assert Sum(-2, 2).add() == 0
  assert Sum(10, -5).add() == 5
  assert Sum(0, 0).add() == 0
  assert Sum(-1, -1).add() == -2
output: 
  def __init__(self, x, y):
      self.x = x
      self.y = y
      
  def add(self):
      return self.x + self.y
  