what is wrong here trying to make my player take damage by a projectile

t$$anonymous$$s is the error Assets\HealthBar.cs(30,4): error CS0201: Only assignment, call, increment, decrement, await, and new object expressions can be used as a statement

t$$anonymous$$s is the code

using System.Collections; using System.Collections.Generic; using UnityEngine; using UnityEngine.UI;

public class HealthBar : MonoBehaviour { public float health; public Slider slider; public Gradient gradient; public Image fill;

  public void SetMaxHealth(int health)

  {         slider.maxValue = health;

      slider.value = health;

      fill.color = gradient.Evaluate(1f);

  }

  public void SetHealth(int health)     {

      slider.value = health;

      fill.color =

gradient.Evaluate(slider.normalizedValue); } void OnCollisionEnter(Collision obj) { if (obj.gameObject.tag=="Projectile") health - health - 1f; }

}