input: 
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          return self.x + self.y
  
  # Example usage:
  # s = Sum(3, 4)
  # print(s.add())  # Output should be 7
  
output:     
      def subtract(self):
          return self.x - self.y
  
  # Example usage:
  # s = Sum(3, 4)
  # print(s.subtract())  # Output should be -1
  