$$(K-M)+(2K-N)+(N+2M)+(M-K)$$
$2\left(K+M\right)$
$$3K-M-N+N+2M+M-K$$
$$3K-M+2M+M-K$$
$$3K+M+M-K$$
$$3K+2M-K$$
$$2K+2M$$
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$$2\left(K+M\right)$$