Formula of nickel oxide with metal deficiency defect in its crystal...

Formula of nickel oxide with metal deficiency defect in its crystal is $\mathrm{Ni}_{0.98} \mathrm{O}$. The crystal contains $\mathrm{Ni}^{2+}$ and $\mathrm{Ni}^{3+}$ ions. The fraction of nickel existing as $\mathrm{Ni}^{2+}$ ions in the crystal is