$\forall$$n$:$\mathbb{N}$, $f$:(\{0..$n$$^{-}$\}$\rightarrow\mathbb{Z}$). ($\forall$$x$:\{0..$n$$^{-}$\}. 0 $\leq$ $f$($x$)) $\Rightarrow$ (0 $\leq$ sum($f$($x$) $\mid$ $x$ $<$ $n$))