Counting from 20 - 1 on every slide

Hello guys, i need you help.
I would like to have a counting window/ number on my Quiz slides. The learner should have only 20 Seconds time. Idea would be: Every quiz slide count down from 20 down to zero in a small window. After reaching the zero the slide change to the next. I tried to make this with the normal variables but i doesn´t work. Is that even possible with normal variables ? And if not - with java and then how?
Thank you in advance for your help!

Eva