Why is this scss output is not showing the calculated total but a string of the var instead?

Question

I have this .scss file / sass:

$var-1:100px;
$var-2:100px;
$var-3:100px;
$total-height:$var-1 + $var-2 + $var-3;
.some-class{
  max-height: calc(100vh - $tota