Let H₁(z) = (1 − pz⁻¹)⁻¹, H₂(z) = (1 − qz⁻¹)⁻¹, H(z) = H₁(z) + rH₂(z). The quantities p, q, r are real numbers. Consider p = 1/2, q = −1/4, |r| < 1. If the zero of H(z) lies on the unit circle, then r = ________