formula % * amount

Hi I want to create a formula based on a % figure * an amount - very similar to the probability and expected revenue fields.

The formula I have at the moment is:

${Potentials.Confidence (%)}*${Potentials.Amount}

The syntax is returning incorrect.

Confidence is a pick list. Potentials amount is the total amount of the potential

Hoping someone is able to help.

Thanks

Guy