Expressions in statements must only be executed for their side-effects?

trying to make health script, but it gives me this error “Expressions in statements must only be executed for their side-effects”

var player : GameObject;
var health = 1000;

function Start()
{

}

function Update()
{
	if(health <= 0)
	{
	    Debug.log("you are dead");
	    Destroy(player);
	}
	
	if(health >= 1000)
	{
		health = 1000;
	}
}

function OnCollisionStay2D(coll: Collision2D)
    {
        if(coll.gameObject.tag == "Enemy")
        {
            health - 100; //says the error is on this line, but i don't know why.
        }
    }

You probably want:

health -= 100;

(Which is:- health = health - 100)