BaTiO_3 has perovskite structure. In this structure, Ba^ 2+...

$\mathrm{BaTiO}_3$ has perovskite structure. In this structure, $\mathrm{Ba}^{2+}$ occupies the corners while $\mathrm{Ti}^{+4}$ occupies the centre of the lattice. If the oxide co-ordination number of $\mathrm{Ba}^{2+}$ ion is x , and that of $\mathrm{Ti}^{4+}$ is y , the $\mathrm{Ba}^{2+}$ ion co-ordination number of oxide ion is $z$ and $\mathrm{Ti}^{4+}$ ion co-ordination number of oxide ion is $t$, then what is the value of ( $x+y+$ $z+t) ?$