input: 
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          """
          Calculate and return the sum of two integers.
          >>> Sum(3, 5).add()
          8
          >>> Sum(-2, 2).add()
          0
          >>> Sum(0, 0).add()
          0
          """
          return self.x + self.y
  
output:     return self.x + self.y