title: Subtraction not working in compass scss
body: <p>While ive been loving <a href="http://compass-style.org" rel="noreferrer">compass</a>, I just ran to a confusing issue. I'm trying to subtract one from a value like this:</p>
  
  <pre><code>padding-left:($top-bar-padding*4)-1;
  padding-left:($top-bar-padding*4-1);
  padding-left:($top-bar-padding*4)-1px;
  padding-left:($top-bar-padding*4-1px);
  </code></pre>
  
  <p>I've tried all of those and they all give me:</p>
  
  <pre><code>padding-left: 20px -1px;</code></pre>
  
  <p>It really puzzling me because I have this earlier in the <code>.scss</code> file and it is working fine:</p>
  
  <pre><code>padding:$top-bar-padding*2+2 $top-bar-padding*4;</code></pre>
  
  <p>Any insights on why this is happening?</p>