input: Write a function to create a class named Sum that initializes with two integers, x and y. The class should have a method named add that returns the sum of x and y. Implement the class and test its functionality with the given examples.
  
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          return self.x + self.y
  
  assert Sum(3, 5).add()==8
  assert Sum(-1, 1).add()==0
  assert Sum(-3, -5).add()==-8
  assert Sum(0, 0).add()==0
  assert Sum(10, -10).add()==0
output: class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          return self.x + self.y