input: Write a function to complete the Python class so that it can calculate the sum of two integers. The class should have an init method to initialize the two integers and a method named add to return their sum.
  
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          return self.x + self.y
  
  assert Sum(3, 4).add() == 7
  assert Sum(10, -5).add() == 5
  assert Sum(0, 0).add() == 0
  assert Sum(-3, -7).add() == -10
  assert Sum(2, 2).add() == 4
output: class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          return self.x + self.y