Field Validation syntax error

Hi,

I need some help with the validation syntax that we have in place for one of the fields on opportunity object. The field name is "Core Est Value", right now there is a validation in place which doe not allow user to enter "-ve" value in there but now we have requirement that should allow user to enetr "-ve" value if the Business Unit is "UGSr". How can I tweak the existing validation to include this condition.

Existing Validation : ([<SalesStage>] <> 'Development' OR [<SalesStage>] <> 'Qualification') AND( [<cBusiness_Unit_Value_ITAG>] > 0)

I tweaked it to this : ([<SalesStage>] <> 'Development' OR [<SalesStage>] <> 'Qualification') AND( [<cBusiness_Unit_Value_ITAG>] > 0) AND [{Business Unit}] = 'UGS'