Multhreading- I Am Using Multithreading To Store Data In "List Array" But I Am Getting Different Output Sometime. Help Me To Correct My Code

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Threading;
 
public partial class Threadinf : System.Web.UI.Page
{
    List<int> garage = new List<int>();
    protected void Page_Load(object sender, EventArgs e)
    {
 
        Thread t1 = new Thread(new ThreadStart(mtd1));
        t1.Start();
        Thread t2 = new Thread(new ThreadStart(mtd2));
        t2.Start();
        while (t1.IsAlive == true || t2.IsAlive == true)
        {
            continue;
        }
        foreach (int val in garage)
        {
            Response.Write(val + "---");
            Response.Write("<br />");
        }
        Response.Write("Total Count :" + garage.Count);
    }
 

    public void mtd1()
    {
        for (int i = 1; i <= 2500; i++)
        {
            Thread.Sleep(1);
            garage.Add(i);
 
        }
    }
 
    public void mtd2()
    {
        for (int j = 25001; j <= 5000; j++)
        {
            Thread.Sleep(1);
            garage.Add(j);
        }
    }
}