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