jQuery checkbox auto disable and re-enable

I have my HTML code and my JavaScript code like this (I'm using jQuery)

HTML :

<input type="checkbox" name="type" value="program" id="box-1" /> Desktop Programming
<select name="lang-1" id="select-1" disabled>
    <option value="c">C/C++</option>
    <option value="vb">VB</option>
    <option value="cs">C#</option>
</select><br /><br />

<input type="checkbox" name="type" value="web" id="box-2" /> Web Development
<select name="lang-2" id="select-2" disabled>
    <option value="asp">ASP</option>
    <option value="php">PHP</option>
    <option value="ror">Ruby on Rails</option>
</select><br /><br />

JavaScript (jQuery) :

$(document).ready(function(){
    var x;

    $("#box-" + x).click(function(){
        var $this = $(this);
        if ($this.is(":checked")) {
            $("#select-" + x).prop('disabled', false);
        } else {
            $("#select-" + x).prop('disabled', true);
        }
    });
});

And I want when I checked the programming checkbox, the programming select option (beside the checkbox) is enable. As well as when I checked the web dev checkbox, the web dev select option is enable. But my code doesn't work. Can anybody help me?