Need help with calculating half damage on previous roll

I am in the process of creating a cloudkill trap for my players. I haven't been able to calculate the half damage for a successful save. nI don't know what I am doing incorrectly. Any help would be appreciated.  Thank you. &{template:default} {{name=Its a Trap!}} {{=The room is filled with a yellow-green gaseous cloud. Make a Con Save DC 15 Failed Save takes [[5d8]]  poison damage! Successful save takes half damage [[$[[0]]/2]] }} !Spawn --name|Cloudkill {{ --offset| 0,1 --light| 1,-5 --size| 8,8 --layer| object --order| toBack }}

You cannot reuse rolls in that way - a roll reference (such as $[[0]]) cannot be used in future calculations (except for some very specific niche cases). You can read more about reusing rolls here . However, you can do something like this (untested): &{template:default} {{name=Its a Trap!}} {{=The room is filled with a yellow-green gaseous cloud. Make a Con Save DC 15 Failed Save takes $[[0.computed]] poison damage! Successful save takes half damage [[ [[5d8]]/2 ]] }} !Spawn --name|Cloudkill {{ --offset| 0,1 --light| 1,-5 --size| 8,8 --layer| object --order| toBack }}

Jarren, Thank you for your help. Your suggested change works well, though I did add the round function to the dice roll calculation, in order to avoid any decimals. Thanks again!