If (1 + x - 2x<sup>2</sup>)<sup>6</sup> = 1 + a<sub>1</sub>x + a<sub>2</sub>x<sup>2</sup> + ... + a<sub>12</sub>x<sup>12</sup>, then a<sub>2</sub> + a<sub>4</sub> + ... a<sub>12</sub> is?
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