Is there any way to know within a validation rule if the record is being evaluated is inserted (first time ) or updated ?

Example: I have this formula in a Validation Rule:

Total_Montos_Financiamiento__c -  PRIORVALUE(Monto__c) +  Monto__c >   Proyecto__r.Importe_Regla_Validacion__c

But when the record is being inserted for fist time, the system dont have PRIORVALUE, but the same amount I 'm trying to save and I want that the PRIORVALUE will be 0 (not the Amount) .

Any help? please!