input: 
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          """
          Calculate the sum of two integers.
          >>> s = Sum(5, 3)
          >>> s.add()
          8
          >>> s = Sum(0, 0)
          >>> s.add()
          0
          >>> s = Sum(-2, -4)
          >>> s.add()
          -6
          """
          return self.x + self.y
  
output:     
          return self.x + self.y
  