input: 
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
      
      def add(self) -> int:
          """ Calculate the sum of two integers.
          >>> s1 = Sum(2, 3)
          >>> s1.add()
          5
          >>> s2 = Sum(-1, 1)
          >>> s2.add()
          0
          """
          return self.x + self.y
  
output:     return self.x + self.y