input: Write a function to implement the `Sum` class so that it can calculate the sum of two integers. The class should have an `__init__` method to initialize the two integers, and an `add` method to return their sum. If the `add` method is called on an instance of the class, it should return 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(2, 3).add() == 5
  assert Sum(-1, 1).add() == 0
  assert Sum(0, 0).add() == 0
  assert Sum(10, -5).add() == 5
  
output: class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          return self.x + self.y