input: Write a function to implement a class named Sum that can calculate the sum of two integers. The class should have a constructor to initialize two integer attributes and a method to return their sum. If the class is instantiated with the integers 3 and 4, the method should return 7.
  
  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, -3).add() == -6
  assert Sum(-2, 5).add() == 3
output: class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          return self.x + self.y