input: Write a function to implement a Python class that can calculate the sum of two integers. The class should have an initializer method to take two integer inputs 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(2, 3).add() == 5
  assert Sum(10, -5).add() == 5
  assert Sum(0, 0).add() == 0
  assert Sum(-3, -3).add() == -6
  assert Sum(1, 0).add() == 1
output: class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          return self.x + self.y