-= help

yep another question.
Im makeing a game where you wander the desert along the noncopyrighted "Silk path".
I have a waterlevel and it is a textnumber.
Now when the actor is created it creates a timer and when the timer goes off its supposed to subract 2.
this is the code i use
waterlevel.textNumber = waterlevel.textNumber -= 2;
But when i do that the textnumber becomes -2.
I tried to correct the code, but then it equals -1.
is there something wrong with the code?
Im makeing a game where you wander the desert along the noncopyrighted "Silk path".

I have a waterlevel and it is a textnumber.
Now when the actor is created it creates a timer and when the timer goes off its supposed to subract 2.
this is the code i use
waterlevel.textNumber = waterlevel.textNumber -= 2;
But when i do that the textnumber becomes -2.
I tried to correct the code, but then it equals -1.
is there something wrong with the code?