input: Write a function to complete the Python class `Sum` so that it can calculate the sum of two integers. The class should include an `add` method that returns the sum of the two integers initialized in the class constructor.
  
  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(-3, -2).add() == -5
output: def add(self):
      return self.x + self.y