title: how to use html input to subtract total value
body: <p>Please check >>><a href="http://codepen.io/anon/pen/zAFfv?editors=101" rel="nofollow">codepen</a> &lt;&lt;&lt;&lt; </p>
  
  <p>I cant get the <em>discountvalue</em> input field to subtract from grandtotal. </p>
  
  <pre><code>&lt;input  id="discountvalue" type="number"&gt; 
  </code></pre>
  
  <p></p>
  
  <pre><code>    function recordToFilename() {
      var input = document.getElementById('discountvalue'),
      discount12 = input.value;
  </code></pre>
  
  <p><br> <em>discount12</em>  does not subtract from grand total. Please check >>><a href="http://codepen.io/anon/pen/zAFfv?editors=101" rel="nofollow">codepen</a> &lt;&lt;&lt;&lt; </p>
  
  <pre><code>document.getElementById('grandtotal').innerHTML = ": $" + (salesTotal * 0.13 + salesTotal - discount12).toFixed(2);
  </code></pre>