input: Write a function to complete the Python class so that it can calculate the sum of two integers. The class should have an `add` method that returns the sum of the two integers provided during initialization. If the class is initialized with integers 3 and 5, the `add` method should return 8.
  
  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(0, 0).add() == 0
  assert Sum(-1, 1).add() == 0
  assert Sum(-3, -5).add() == -8
  assert Sum(10, 10).add() == 20
output: class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          return self.x + self.y