input: Write a function to complete the Python class so that it can calculate the sum of two integers. The function should return the sum of the two integer attributes of the class. If the class is instantiated with attributes x and y, the function should be able to return the result of x + y.
  
  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(-1, 1).add() == 0
  assert Sum(-5, -3).add() == -8
  assert Sum(10, -10).add() == 0
output: def add(self):
      return self.x + self.y