How to make a negative condition

[Aawqq2112]

How do I make a negative condition in this script, I need not instead of on, but it throws an error

on walking on dirt or grass:
   remove slowness from player

 

[GooMooPunch]

You could just make it so you only get slowness while walking on dirt or grass.

on walking on dirt or grass:
    if {uuid::slow%player%} is false:
        apply slowness to player for 1 second
        set {uuid::slow%player%} to true
        wait 0.999999 seconds
        set {uuid::slow%player%} to false
    else if {uuid::slow%player%} is not set:
        apply slowness to player for 1 second
        set {uuid::slow%player%} to true
        wait 0.999999 seconds
        set {uuid::slow%player%} to false
    else:
        stop