Video Background Duration

Hi all,
I insert into my Storie a default video as a background. The problem is that this video last 33 seconds and makes that storie page a 33 second long. Can I cut the time of the background video inside the tool? Or I need to do this with an external tool?

Thank you all.

PD: As you may ask, I turn ON automatic forward Stories, with a 6 seconds duration, but the video duration overrides this.