[Zope] Simple calculation

I am obviously thick. ;-) I want to do a simple calculation in a dtml tag and print the result: <dtml-var expr="85 * HowOften"> HowOften is a value that has been set by an HTML input in a web form. When I run this with a value of 3 for the result is that it prints the character 3 85 times! So it treats it as a string. Obviously I need to convert HowOften to a float, but how do I do it? Cheers Marc