My meta box (custom fields) won't save…

Hi,

I don’t understand which mistake I’ve done. I’ve created a custom post type and I have also set a custom field for them. But I can’t save the custom field, here is my code (I’m not a programmer)… anyone can help me?

add_action( 'add_meta_boxes', 'custom_field_tetti' );

function custom_field_tetti(){
	add_meta_box("specifiche_tetti_meta", "Specifiche Tetti", "specifiche_tetti", "tetti", "normal", "high" );
}

function specifiche_tetti(){
	global $post;
	$custom_seriale = get_post_custom($post->ID);
	$tetti_seriale = $custom_seriale["tetti_seriale"][0]; ?>
	<label>Seriale:</label>
	<input name="tetti_seriale" value="<?php echo $tetti_seriale; ?>"/>

<?php }

//SALVO I CAMPI PRESONALIZZATI DEL CUSTOM POST TYPE TETTI
add_action('save_post', 'save_specifiche_tetti');
global $post;
function save_specifiche_tetti(){
	update_post_meta($post->ID, "tetti_seriale", $_POST["tetti_seriale"]);
}

?>