For three positive integers \(\rm p, q, r, x^{p q^2}=y^{q r}=z^{p^2 r}\) and r = pq + 1 such that 3, 3log_y x, 3 log_z y, 7 log_x z are in A.P. with common difference \(\frac{1}{2}\). Then r – p – q is equal to