Set Value Formula

i am using 8.2.2

i am trying to set the value of a number field called Lama Bekerja based on the months inputted in Mulai Bekerja accounting the year aswell, i know that using business rules where you can use set value = formula like so...

now when i open it, it doesn't let me do the calculations

can someone explain a method on how to implement this in creatio 8.2.2?