input: Write a function to complete the Python class so that it can calculate the sum of two integers.
  
  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, 1).add() == 0
  assert Sum(0, 0).add() == 0
  assert Sum(-5, -3).add() == -8
  assert Sum(10, -10).add() == 0
output: def add(self):
      return self.x + self.y