Collision damage in C#

I’m very new to Unity and i’m trying to make a top-down space shooter. My collision script doesn’t work like i thought. Im trying to decrease enemy health but it gives me this error Assets/CollisionDamage.cs(12,33): error CS0201: Only assignment, call, increment, decrement, and new object expressions can be used as a statement

using UnityEngine;
using System.Collections;

public class CollisionDamage : MonoBehaviour {
	public int health = 1;
	public int damage = 1;
	void OnTriggerEnter2D(){
		Debug.Log ("Trigger");

	void Update() {
		if (health <= 0) {
			Die ();

	void Die() {
		Destroy (gameObject);

as the error states, line 12 is the problem. that’s not valid syntax in any language.

you probably mean:

health -= damage;