input: from typing import Type
  
  def calculate_sum(cls: Type[Sum]) -> int:
      """Complete the given Python class 'Sum' so that it can calculate the sum of two integers.
      >>> sum_instance = Sum(2, 3)
      >>> calculate_sum(sum_instance)
      5
      >>> sum_instance = Sum(4, -1)
      >>> calculate_sum(sum_instance)
      3
      """
output:     return cls.a + cls.b