How to add extra values in already changed value?(Missing Viewstate probably)

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C#

ASP.NET

Hi everybody,
I have a label with a 90 value .When I click a button the label becomes 80 70 or 60,since it -= a variable connected to that label.It always counts the 90 and can't go below 80 70 60.This is the code :

public partial class Default : System.Web.UI.Page
    {
        Powers powers = new Powers();
        int airElementalHealth = 90;
        int earthElementalHealth = 120;

        protected void Page_Load(object sender, EventArgs e)
        {
                airelementalHealthLabel.Text = airElementalHealth.ToString();
            ViewState.Add("airElementalhealth", health(airElementalHealth));
        }

        protected void usePowerButton_Click(object sender, EventArgs e)
        {
            //airElementalHealth = health(airElementalHealth);
           
                airelementalHealthLabel.Text = health(airElementalHealth).ToString();
            
        }

        private int health(int defendersHealth)
        {
                Random random = new Random();
                int damage = powers.powersArray[random.Next(3)];//It works but the health doesn t drop continuously after button clicks
                //the value changes always according int airElementalHealth = 90;
                defendersHealth -= damage;
            
            return defendersHealth;
        
        } 
    }
}

Thank you a lot for your time.