Missing Variable

I have a page that pass one variable to other page, this variable can have 4 possible values :

S; M; L and XL

I want to prevent that the destination page is only shown if a variable is passed together. Some people can try to not pass the variable:

I have the following :

 if (!Page.IsPostBack) {
 if (Request.QueryString["Plano"] == "S") {
 OK the page is shown
 }
 if (Request.QueryString["Plano"] == "M") {
 OK the page is shown
 }
 if (Request.QueryString["Plano"] == "L") {
 OK the page is shown
 }
 if (Request.QueryString["Plano"] == "XL") {
 OK the page is shown
 }
 if (Request.QueryString["Plano"] == "") {
 Response.Redirect ("default.aspx");
 }

The problem is that I can see the page if I put ... page.aspx without the "Plano" variable.

How can I prevent this ?

 if (!Page.IsPostBack) {
 bool OK=false;
 if (Request.QueryString["Plano"] == "S") {
 //OK the page is shown
 OK=true;
 }
 if (Request.QueryString["Plano"] == "M") {
 //OK the page is shown
 OK=true;
 }
 if (Request.QueryString["Plano"] == "L") {
 //OK the page is shown
 OK=true;
 }
 if (Request.QueryString["Plano"] == "XL") {
 //OK the page is shown
 OK=true;
 }
 if (OK!=true) {
 Response.Redirect ("default.aspx");
 }

Thank you.

Why complicate simple things ;-)

Best regards.
Might want to use an OR in there. Just a thought :-)

If plano = "S" or pano = "XL" ... then

else

end if