Answer: i was need to use int's or floats without symbols, and the best way was to check WooCommerce docs here: Woo - Docs , and to search for this variable and use it without () (not function). now it work :)
<?php
$first_number = $woocommerce->cart->subtotal;
$second_number = $woocommerce->cart->discount_total;
$sum_total = $first_number - $second_number;
print ($sum_total);
?>