input: Write a function to complete the Python class Sum so that it can calculate the sum of two integers. The function should define the method `add` in the class Sum which returns the sum of the two integers `x` and `y`.
  
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
  
      def add(self):
          return self.x + self.y
  
  assert Sum(5, 10).add() == 15
  assert Sum(-3, 3).add() == 0
  assert Sum(0, 0).add() == 0
output: 
  def add(self):
      return self.x + self.y
  