The oxidation states of transition metal atoms in $\mathrm{K}_2 \mathrm{Cr}_2 \mathrm{O}_7, \mathrm{KMnO}_4$ and $\mathrm{K}_2 \mathrm{FeO}_4$, respectively, are $\mathrm{x}, \mathrm{y}$ and z . The sum of $\mathrm{x}, \mathrm{y}$ and z is $\_\_\_\_$ .
