input: Write a function that defines a class `Sum` which initializes with two integers `x` and `y`, and has a method `add()` that returns the sum of these 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(10, -5).add()==5
  assert Sum(0, 0).add()==0
  assert Sum(-3, -3).add()==-6
  assert Sum(123, 456).add()==579
output: class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          return self.x + self.y