How to totalise time formatted cells

I know the title is stupid...
I am making a timesheet for my employees. On the sheet I have to put in when the arrive and when they quit. I have the time format 00:00 on the cells.

For ex. one arrives at 08:00 (cell A1) and leaves at 15:45 (cell A2).
What formula do I put in A3 so the outcome will be 7,75 hours?