A disk of radius R has positive charge uniformly distributed over an inner circular region of radius a and negative charge uniformly distributed over the outer annular (ring-shaped) region . The surface charge density on the inner region is +σ, and that on the annular region is −σ. The electrostatic potential at P, located on the central perpendicular axis a distance z=R from the disk center, is zero.

What is a?

Express your answer in terms of R. Express the coefficients using three significant digits.