Calculating time overlap

Hello!

I need help with a formula to calculate overlapping observation time. We observe multiple subjects (3 or 4 at one time) but each subject comes in and out of view multiple times throughout the total observation time. I am can not figure out a formula to find out the total time that subject 1 and 2, 1 and 3, 1 and 4, 2 and 3...etc are both observed at the same time.

I have attached a sample of the times observed...

Thank you!