Metal oxidation numbers: Ni in $\mathrm{Ni}(\mathrm{CO}){}_4$ (CO neutral), Fe in $\mathrm{K}{}_4[\mathrm{Fe}(\mathrm{CN}){}_6]$, Fe in $\mathrm{K}{}_3[\mathrm{Fe}(\mathrm{CN}){}_6]$, Ni in $\mathrm{K}{}_2[\mathrm{Ni}(\mathrm{CN}){}_4]$ (cyanide $-1$, K $+1$). What is the sum of these four oxidation numbers?

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