$$(12+15)-(3+2+1)-4-(1+2-6+7)$$
$13$
$$27-\left(3+2+1\right)-4-\left(1+2-6+7\right)$$
$$27-\left(5+1\right)-4-\left(1+2-6+7\right)$$
$$27-6-4-\left(1+2-6+7\right)$$
$$21-4-\left(1+2-6+7\right)$$
$$17-\left(1+2-6+7\right)$$
$$17-\left(3-6+7\right)$$
$$17-\left(-3+7\right)$$
$$17-4$$
$$13$$
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