Third custom field as result of the values of two other fields
I want to have a field where the result of a fundraisin action is calculated. The first value is that of the amount of money I requested to a fund for the realisation of a theater production, e.g. € 5000. Suppose the fund gives 4000.
I want the result: - € 1000 automatically shown in the third field. Is it possible to make a formula to do so?
(Insightly has the possibility to use calculated fields, but they are only available in the Enterprise plan, and that should be the only reason to take such a plan. So that's no option, but is there another way?)