input: 
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
      
      def add(self):
          """ Calculate the sum of two integers.
          >>> Sum(5, 3).add()
          8
          >>> Sum(-2, 2).add()
          0
          >>> Sum(10, -15).add()
          -5
          """
          return self.x + self.y
  
output:     return self.x + self.y