Where is this (-1) coming from in my change level script?

I have a script that changes from Level 1-1 to level 1-2 just fine.

   function OnTriggerEnter (other : Collider){
      if(other.gameObject.tag == "Player"){
         Application.LoadLevel("1-2");
         }
         }

But I have a script that is exactly the same but to loads level 1-3 instead.

   function OnTriggerEnter (other : Collider){
      if(other.gameObject.tag == "Player"){
         Application.LoadLevel("1-3");
         }
         }

But when I enter the trigger I get this error:

Level ‘1-3’ (-1) couldn’t be loaded because it has not been added to the build settings.
To add a level to the build settings use the menu File->Build Settings…
UnityEngine.Application:LoadLevel(String)
Level 1-3:OnTriggerEnter(Collider) (at Assets/Scripts/NextLevels/Level 1-3.js:5)

Where is the (-1) Coming from? I have level 1-3 in the Scenes in build menu.
The same thing was happening when I was trying to go from 1-1 to 1-2 but it just stopped randomly.

Thanks in advance for any help.

The -1 comes is probably the internal return value for the LoadLevel method indicating an error. It is classic in C++ to have a negative value for fail (in any language actually).

In your case, it indicates what is said after, the scene is missing from the build. Open up the Build setting, I think it is in File, then drag and drop the missing scene there.