input: 
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          return self.x + self.y
  """
  Add a method to the 'Sum' class that returns the sum of two integers.
  
  Example:
  >>> s = Sum(3, 4)
  >>> s.add()
  7
  """
  
output:     
      def subtract(self):
          return self.x - self.y
  