43. Let $H_1(z)=(1-pz^{-1})^{-1},H_2(z)=(1-qz^{-1})^{-1},H(z)=H_1(z)+rH_2(z)$ . The quantities p, q, r are real numbers. Consider $p={1\over2},q={1\over4},|r|<1$ If the zero of H(z) lies on the unit circle, then
r = ________