I tried to get session for my login page to content page, when I logout and press back button it redirects to my content page

Question

My login page code:

protected void Page_Load(object sender, EventArgs e)
{
    Response.Cache.SetCacheability(HttpCacheability.NoCache);
    if (!Session["userid"].Equals(String.Empty))
    {
        Response.Redirect("Welcome.aspx");
    }
}
protected void CheckBox2_CheckedChanged(object sender, EventArgs e)
{

}
protected void LinkButton1_Click(object sender, EventArgs e)
{
    Response.Redirect("userreg.aspx");
}

Content page code:

protected void Page_Load(object sender, EventArgs e)
{
    if (Session["userid"].Equals(String.Empty))
    {
        Response.Redirect("Default.aspx");
              }
    Response.Cache.SetCacheability(HttpCacheability.NoCache);
}
protected void Button1_Click(object sender, EventArgs e)
{
    Session.Abandon();
    Response.Redirect("Default.aspx");
}