Formula to find lowest time n 2 groups then the difference between them ignoring “-“

So I need to have a formula look to F, G, H, I.

look to see which is a lower time

F or G

then look to see which is a lower time in

H or I

then subtract the lower time from F/G from the lower time in H/I.

it will also need to ignore any "-"- it encounters

F g h i

AMB Fire AMB12L Fire12L

23:15:09 23:19:18 23:00:00 23:20:00
23:19:18 23:15:09 23:20:00 23:00:00
23:19:18 23:15:09 - 23:00:00
23:19:18 23:15:09 23:20:00 -


Any help would be greatly appreciated.