Several videos in a unit to mark unit complete

Hi, I need to add several videos on a unit and in order to "mark unit complete" the students need to watch all videos in a single unit to access to next unit. I've managed to lock the next unit for one only video. But when I have more that one video to complete the unit, even if only one is read student can have access to the "mark unit complete" button.. but they shoudn't. (they need watch all videos). How can i do that please ? Thanks for your help !!