get shortcode value into php snippet

Hello i need to add simple math operation. this is code

$a = 2;
$b = 8;

echo $a / $b *100;

but i need to add there vaues of shortcode like

$a = <?php echo do_shortcode('[name_of_shortcode]'); ?>
$b = <?php echo do_shortcode('[name_of_shortcode2]'); ?>

but this does not work. Could you please help me find correct way?

Thanks in advance