How to design a control when you select any 3 of 4 parameters to get the remaining one

I am thinking from the UX perspective about implementing a control where there are four parameters (see example below). The user can select the values for any three of them to get the value of the fourth (parameters are related).

Example

The 4 parameters are E (energy), V (voltage), I (current), t(time). When I select any 3 out of the 4 - the last one is obtained from the equation - E = V x I x t (energy = voltage x current x time).