SUM function

[ResolutionsNET]

Can anyone help, new to XSL.

<a number1=&quot;3&quot;>
<b number2=&quot;2&quot; /b
</a>
<a number1=&quot;5&quot;>
<b number2=&quot;4&quot; /b
</a>

I would like to sum to number1 * number2.

e.g. (3*2) + (5*4)

Any ideas.