def METHOD_NAME(
    undiscounted: TaxedMoneyRange, discounted: TaxedMoneyRange
) -> Optional[TaxedMoney]:
    """Calculate the discount amount between two TaxedMoneyRange.
    Subtract two prices and return their total discount, if any.
    Otherwise, it returns None.
    """
    return _get_total_discount(undiscounted.start, discounted.start)