$$12-(-4+25-6)+(-6+7-15)+2$$
$-15$
$$12-\left(21-6\right)-6+7-15+2$$
$$12-15-6+7-15+2$$
$$-3-6+7-15+2$$
$$-9+7-15+2$$
$$-2-15+2$$
$$-17+2$$
$$-15$$
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