input: Write a function to complete the given Python class so that it can calculate the sum of two integers. The function should define the add method within the class to return the sum of the instance variables x and 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(5, -2).add() == 3
  assert Sum(0, 0).add() == 0
  assert Sum(-1, -1).add() == -2
  assert Sum(-5, 7).add() == 2
output: 
  def add(self):
      return self.x + self.y
  