Let C be a circle with center P, and AB be a diameter of C. Suppose P1 is the midpoint of the line segment PB, P2 is the midpoint of the line segment P1B, and so on. Let C1, C2, C3, C4, ... be circles with diameters P1P2, P2P3, P3P4, ... respectively. Suppose the circles C1, C2, C3, ... are all shaded. The ratio of the area of the unshaded portion of C to that of the original circle is