Burndown Chart 20%

Hi,

I read that when we do the Sprint planning we have to put additional items in the Sprint Backlog in case we finish sooner and I would like to know:

Do we have to count those items in our BurnDown Chart ?

Example : In my Sprint Backlog we have 20 hours of work and the additional items count 2h (which are in the 20 hours)

Do we have to start with 20 hours or 18 hours ?