Something with mousescreeny

[GoZelda]

I'm having a bit of trouble with using mousescreeny.

NPC Code:

---

    if (leftmousebutton){

    if (mousescreenx in |0,20| && mousescreeny in |screenheight,screenheight-20|){

      message wtf.;

    }

  }

---

The part in red is what goes wrong. It works if I enter variables such as 1080,1100. But whenever I try using screenheight, it won't work. Someone care to explain what I'm doing wrong and what should be entered here?