Miller's Theorem - Input Capacitance
In general case the situations look like this 



Now let as try to find a input resistance.

Rin = Vin/Iin

In = (Vin - Vout)/R = (Vin - A*Vin)/R = Vin * (1 - A)/R

Rin = Vin/Iin = R/(1 - A) 

As you can see we have a "minus" sign. 
We get the "plus" sign only when our amplifier gain is negative (inverting amplify) 

Rin = R/(1 - (-A)) = R/(1+|A|)