input: Write a function to create a Python class named "Sum" that has an initialization method accepting two integers 'x' and 'y' as parameters. The class should also have a method named 'add' which returns the sum of the two integers. If the class is correctly implemented, the following assertions should pass:
  
  assert Sum(3, 4).add() == 7
  assert Sum(-1, 5).add() == 4
  assert Sum(0, 0).add() == 0
  assert Sum(100, -100).add() == 0
  assert Sum(-3, -7).add() == -10
output: class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
      
      def add(self):
          return self.x + self.y