Help with function to update with same value

Help with function to update with same value

Hello, 

I need help with this function. 

in deals module, I have tree fields:
- "Respuesta Cliente" (List)
- "Plazo entrega CPQ dias" (Decimal)
- "PLAZO CONTRATO" (Number)

I have to create a workflow rule, that when "Respuesta Cliente" containts "Agree" or "Long-Term", then a function is activated to update "PLAZO CONTRATO" with same value as "Plazo entrega CPQ dias".

Thank you