[jQuery] Var from Ajax success not defined?

[heohni]

Hi,

$.ajax({
                type: "POST",
                url: "/ajax_add_selected_product_to_session.php",
                success: function(msg){
                        /* add here what to do on success */
                        //alert(msg);
                        var how_many_checked = msg;

                }
        });
alert(how_many_checked);
if(how_many_checked > 0) .......


the FF returns "how_many_checked" ist not defined.
Why?

How can I define a var and use it for a later coming if / else part?

Thanks a bunch!