input: Write a function to complete the Python class `Sum` so that it can calculate the sum of two integers. Implement a method `add` within the class that returns the sum of the instance variables `x` and `y`.
  assert Sum(1, 2).add()==3
  assert Sum(0, 0).add()==0
  assert Sum(-1, 1).add()==0
  assert Sum(10, -5).add()==5
  assert Sum(-3, -3).add()==-6
output: class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
      
      def add(self):
          return self.x + self.y