input: Write a function to complete the python class so that it can calculate the sum of two integers. The class should have an __init__ method to initialize two attributes x and y, and a method named add that returns the sum of x and y.
  assert Sum(1, 2).add()==3
  assert Sum(0, 0).add()==0
  assert Sum(-1, 1).add()==0
  assert Sum(5, -3).add()==2
  assert Sum(-5, -3).add()==-8
output: class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
      
      def add(self):
          return self.x + self.y