How to get Value

How to get the Value of last selected option of HTML Multi Select ListBox using jQuery,

When I select "Item 1 and Item 3" and click button "Add to Cart" system select big value "3",
How to get the value selected item like this,
Example : Select Item 1 & Item 4 and get value "2".

Thanks you....

PHP Code:

<?php
    echo form_open('', array(
        'method' => 'post',
        'class' => 'sky-form',
        'id' => 'demoform',
        'style' => 'border:none',
    ));
?>

<select id="selectList" multiple="multiple" size="10" name="qty">
    <option value="1">Item 1</option>
    <option value="2">Item 2</option>
    <option value="3">Item 3</option>
    <option value="4">Item 4</option>
</select>

<script>
var demo1 = $('[name=qty]').bootstrapDualListbox();
$('#selectList option:selected').val();
</script>

<button type="button" class="btn add_to_cart" data-type="text"  data-pid='<?php echo $row['product_id']; ?>'>
        <i class="fa fa-shopping-cart fa-fw"></i>
        <?php echo translate('add_to_cart'); ?>
</button>

</form> 

Source :
http://www.jqueryscript.net/form/Respons...stbox.html

Old script
http://www.jqueryfaqs.com/Articles/Get-t...Query.aspx