How to convert string to number in expression builder

Hi,

We unfortunately use a short text field to allow users to input an amout.

Now we need to set up workflow rules to validate that the value users filled in is bigger than 1000. 

However, we can not find any expression builder function which can help convert the short text string to a number to directly compare with 1000.

Could any expert suggest a workable solution? Many thanks.

Neil Li,

Best Regards