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");
         
         health-damage;
     }
 
     void Update() {
         if (health <= 0) {
             Die ();
         }
     }
 
     void Die() {
         Destroy (gameObject);
     }
 }