PowerPoint Countdown not working properly

Hello community,

I have a powerpoint presentation with a countdown timer.
If you open the presentation, you will see on the top left the countdown and on the top right a number for seconds (I can change any number of seconds I want, and it will use the number as countdown.

What I would like to achieve is the following.
When I open the powerpoint in slideshow mode, the countdown should start right away.
If I press Enter to go to the next slide, the countdown should start again with the number of seconds given on the top right.
Can you help me with this?
Thank you.