input: Write a function to create a Python class that can calculate the sum of two integers. The class should have an initialization method that takes two integers and a method named 'add' that returns 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(-1, 5).add() == 4
  assert Sum(0, 0).add() == 0
  assert Sum(10, -2).add() == 8
  assert Sum(-3, -3).add() == -6
output: class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          return self.x + self.y