input: 
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          """ Calculate and return the sum of two integers.
          >>> Sum(1, 2).add()
          3
          >>> Sum(5, 7).add()
          12
          """
          return self.x + self.y
  
output:     
          return self.x + self.y
  