How can I use a case statement to calculate trip duration?

end_datetime - start_endtime = trip duration.
If the trip duration is less than 0 then I want to do:
start_endtime - end_datetime = trip duration.

Here is the code I'm trying to use:

Select case when datediff(minute, started_at, ended_at) < 0 
then 
	datediff(minute, ended_at, started_at)
else 
datediff(minute, started_at, ended_at) 
 end
 
 FROM detail

Thanks,
David