I am trying to add a new row in a table via javascript. 
I want to have the row inserted before the last row where the sum is, and also the number in the label to be incremented with each new row inserted in a table. 

```
<input type=button value='Add Row' />

<table cellspacing=0 cellpadding=4 id="table">
<tr><td>Number 1:</td><td><input value=20 style="width:30px" type=text  maxlength=2/></td></tr>
<tr><td>Number 2:</td><td><input value=25 style="width:30px" type=text  maxlength=2/></td></tr>
<tr><td style="border-top:solid 1px black;border-bottom:solid 1px black;">Sum:</td><td style="border-top:solid 1px black;border-bottom:solid 1px black;"><div>45</div></td></tr>
</table>

```

That's the code for the table, I want to have it under 'number 2' row, but before the sum row. 
And with each new row the label to be changed to Number 3: for example or Number 4, etc.
Any idea how I can do that. I need to use only javascript and no external tools or jquery is allowed.
Thanks in advance, Laziale