I want to execute a command in which within a certain radius, players will always be in adventure mode, once outside of that radius, they will be in survival. The command I am trying to use is /gamemode a @a [rm=100] Gamemode S but it's not working quite right.

How do I do this? I do not have much experience with command blocks, all help is appreciated!

Edit: The link provided does not work with a command block, anything else?