input: Write a function to complete the Python class `Sum` so that it calculates the sum of two integers correctly. The class should have an `__init__` method to initialize the objects with `x` and `y`, and an `add` method to return the sum of `x` and `y`.
  assert Sum(2, 3).add()==5
  assert Sum(10, -5).add()==5
  assert Sum(0, 0).add()==0
  assert Sum(-1, -1).add()==-2
  assert Sum(5, 0).add()==5
output: class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
      
      def add(self):
          return self.x + self.y