Field Validation

Hi all,

I have to define a field validation rule.

According to this rule,

If my custom currency field is negative, then pick list field should have any one of the specified values selected. How do I go about creating the rule ?

Example:

Sales stage pick list values-1,2,3,4,5

Rule:

If

Revenue-currency field- negative

Then

Sales stage-pick list- any of the following values should be selected (1,3,5)