input: Write a function to define a Python class that can calculate the sum of two integers. The class should have an initialization method to store the two integers as attributes and a method 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(0, 0).add() == 0
  assert Sum(-2, 2).add() == 0
  assert Sum(100, -50).add() == 50
  assert Sum(-1, -1).add() == -2
output: class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          return self.x + self.y