title: NaN error when multiplying
body: <p>I have this script to multiply two columns and display total in third. Here it is code:</p>
  
  <pre><code>&lt;script&gt;
  $(function() {
      $("#num1, #num2").on("keydown keyup", sum);
      function sum() {
      $("#total").val(Number($("#num1").val()) * Number($("#num2").val()));
      }
  });
   &lt;/script&gt;
  </code></pre>
  
  <p>And form:</p>
  
  <pre><code>&lt;form method="POST" name="form1" action="" "&gt;
  
  &lt;label&gt;num1&lt;/label&gt; 
  &lt;input name="num1" id="num1" type="text" value="SOMENUMBER" /&gt;
  
  &lt;label&gt;num2&lt;/label&gt;
  &lt;input type="text"   name="num2" id="num2" value="SOMENUMBER"/&gt;
  
  &lt;label&gt;total&lt;/label&gt; 
  &lt;input type="text"  name="total" style="width: 40%;" id="total" readonly /&gt;
  
  &lt;button class="square" name="btn" type="submit"&gt;SOMEBUTTONNAME&lt;/button&gt;
  
  &lt;/form&gt;
  </code></pre>