[SQL] Calculate time difference in PSQL with HH:MM splitted in several columns

I've a PSQL table like this:

Order Start_Hour Start_Minute Finish_Hour Finish_Minute
10 10 15 12 15
10 12 15 14 15
10 16 00 17 00

And I need to calculate by a query the total time expressed in hours that I spent to finish the order. In this scenario I expect to have a total of 5 hours:

12:15 - 10:15 = 2 hours
14:15 - 12:15 = 2 hours
17:00 - 16:00 = 1 hours


The query result must be 5.

The idea was concatenate start hour/minute and finish hour/minute, convert them to hour, make the difference, calculating the total.

SELECT (Start_Hour & ":" & Start_Minute) as start, (Finish_Hour & ":" & Finish_Minute) as finish

FROM OrderDetails


But when I try to convert them to HH:MM using cast or convert but I got errors.

Any advice? Thank you

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