Same input appears and disappears map?

Hi everyone, I'm sure this is a really silly question, but I can't make it work. I want to reveal a map (GUI texture) when pressing the M key, and hide it when pressing the same key again. I'm using javascript with Input.GetKeyDown("m") but once it appears with the M key, I can't make it dissapear with the same key.

Thanks to all, Best.

or

var guiShows : boolean = false;

if (Input.GetKeyDown ('m'))
{
guiShows = !guiShows;
}

this will toggle between true and false.