input: Write a response that appropriately completes the request by creating a method in the Sum class that can calculate the sum of two integers.
  
  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(-1, 1).add()==0
  assert Sum(0, 0).add()==0
  assert Sum(-5, -3).add()==-8
  assert Sum(10, -10).add()==0
output: def add(self):
      return self.x + self.y