C# code help (Forms proj)

I'm trying to make button2 calculate all the items in the list. It worked but when you click calculate button again, it will continue adding. Is there a way to stop that (Newb coder here)?

private void button2_Click(object sender, EventArgs e)
        {
            Total1.Text = "0";
            Boolean isValid = true;
            //(Red) 
           
            //(Blue) 
            
            //(Yellow) 
           

            if (ListBox2.Items.Contains("(Red) "))
            {
               
                Health += 10;
                Total1.Text = Health.ToString();

            }
            else if (ListBox2.Items.Contains("(Red) "))
            {
                Health += 1;
                Total1.Text = Health.ToString();
            }

             // no click
            else
            {
                MessageBox.Show("Error");
            }