input: 
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          """Return the sum of two integers.
          >>> sum_obj = Sum(2, 3)
          >>> sum_obj.add()
          5
          >>> sum_obj = Sum(-1, 1)
          >>> sum_obj.add()
          0
          """
          return self.x + self.y
  
output:     
          return self.x + self.y
  