Имя "z" отсутствует в текущем контексте. Что дописать ? - C#

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
 
namespace WindowsFormsApplication4
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
 
        private void button1_Click(object sender, EventArgs e)
        {
            this.timer1.Start();
            double x, z;
            double p = 3.14;
            x = Convert.ToInt32(textBox1.Text);
            z = p * (Math.Pow(x, 2));
           
        }
 
        private void timer1_Tick(object sender, EventArgs e)
        {
            progressBar1.Increment(10);
            if (progressBar1.Value == 100) {
                textBox2.Text = Convert.ToString(z);
            }
        }
    }
}