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
  s1 = Sum(3, 5)
  print(s1.add())  # Output: 8
  
  # Example 2
  s2 = Sum(-1, 1)
  print(s2.add())  # Output: 0
  
  # Example 3
  s3 = Sum(0, 0)
  print(s3.add())  # Output: 0
  
output:     
  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
  s1 = Sum(3, 5)
  print(s1.add())  # Output: 8
  
  # Example 2
  s2 = Sum(-1, 1)
  print(s2.add())  # Output: 0
  
  # Example 3
  s3 = Sum(0, 0)
  print(s3.add())  # Output: 0
  