input: Write a function to complete the Python class so that it can calculate the sum of two integers. If the class is completed, it should have an 'add' method that returns the sum of the two integers.
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
  
      def add(self):
          return self.x + self.y
  
  assert Sum(3, 4).add() == 7
  assert Sum(-1, 1).add() == 0
  assert Sum(10, -10).add() == 0
  assert Sum(0, 0).add() == 0
  assert Sum(15, 25).add() == 40
output: class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
  
      def add(self):
          return self.x + self.y