calculating fields

I am trying to calculate some fields in my view template. I use the following shortcode function in my functions.php:
add_shortcode('wpv-calculate', 'calculate_shortcode');
function calculate_shortcode($atts,$content=null) {
$content = wpv_do_shortcode($content);
$content = eval("return $content;");
return $number;
}

In the view template:
[wpv-calculate][types field="db-os-reads"][/types]-[types field="db-os-writes"][/types][/wpv-calculate]

but the result is empty.