For three positive integers p, q, r, ${\mathrm{x}}^{{\mathrm{pq}}^2}={\mathrm{y}}^{\mathrm{qr}}={\mathrm{z}}^{{\mathrm{p}}^2\mathrm{r}}$ and $r=pq+1$ such that $3, 3{\log }_y x,3{\log }_z y,7{\log }_x z$ are in A.P. with common difference 1/2. Then   $r-p-q$ is equal to