input: Write a function that completes the given Python class `Sum` so that it can calculate the sum of two integers. The class should initialize with two integers `x` and `y`, and have a method `add` that returns their sum.
  
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          return self.x + self.y
          
  assert Sum(2, 3).add() == 5
  assert Sum(-1, 1).add() == 0
  assert Sum(0, 0).add() == 0
  assert Sum(10, -5).add() == 5
  assert Sum(-3, -3).add() == -6
output: 
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          return self.x + self.y
  