input: Write a function that completes the Python class Sum so that it can calculate the sum of two integers. The function should define the `add` method within the class to perform this calculation.
  
  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, 5).add() == 4
  assert Sum(0, 0).add() == 0
  assert Sum(-2, -2).add() == -4
  assert Sum(10, -3).add() == 7
output: class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          return self.x + self.y