input: 
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          return self.x + self.y
  
  # Test cases:
  # Example 1:
  s = Sum(3, 5)
  print(s.add())  # Expected output: 8
  
  # Example 2:
  s = Sum(-1, 1)
  print(s.add())  # Expected output: 0
  
output:     
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          return self.x + self.y
  