Gaps-and-islands: get time spent in state

I have a table recording states and timestamps, like this one:

And I would need to get the time spent within each state, whereas the end time for each state is the timestamp value of the next record; like this:

And the expected result would be:

I tried using the LAG() and LEAD() window-functions, but could not get the expected result.
Could you please help me?

Thanks a lot in advance!

>Solution :

you can use window function lead() like so:

with cte as (
  select serialno
  , state 
  , lead(timestamp) over (partition by serialno order by timestamp) - timestamp as duration
from DeviceState
)
  
select serialno, state 
  , sum(duration) as duration
from cte 
group by serialno, state

output:

fiddle