input: Write a function to create a Python class named Sum which can calculate the sum of two integers. The class should have an initializer to accept two integers as input and a method named add to return their sum.
  
  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, 5).add()==10
  assert Sum(-1, 1).add()==0
  assert Sum(-3, -4).add()==-7
output: class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          return self.x + self.y