input: 
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          """Return the sum of two integers x and y.
          >>> s = Sum(3, 4)
          >>> s.add()
          7
          >>> s = Sum(-1, 1)
          >>> s.add()
          0
          >>> s = Sum(0, 0)
          >>> s.add()
          0
          """
          return self.x + self.y
  
output:     
          return self.x + self.y
  