input: 
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          """ Add two integers and return their sum.
          >>> Sum(5, 3).add()
          8
          >>> Sum(-10, 10).add()
          0
          """
          return self.x + self.y
  
output:     
          return self.x + self.y
  