The oxidation states of transition metal atoms in K_2 Cr_2 O_7, KMnO_4 and K_2 FeO_4, respectively, are x, y and z. The sum of x, y and z is \ \ \ \.

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