input: def calculate_sum(s: Sum) -> int:
      """ Given an instance of the Sum class, calculate the sum of the two 
      integers stored in the instance.
      >>> s = Sum(3, 4)
      >>> calculate_sum(s)
      7
      >>> s = Sum(10, -5)
      >>> calculate_sum(s)
      5
      """
output:     return s.a + s.b