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EXAMPLE
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The cases of a(4) = 16 are:
[1, 2, 3, 4] * [-4,-1,-2, 3] = -4 - 2 - 6 + 12 = 0.
[1, 2, 3, 4] * [-2,-3, 4,-1] = -2 - 6 + 12 - 4 = 0.
[1, 2, 3, 4] * [-3,-4, 1, 2] = -3 - 8 + 3 + 8 = 0.
[1, 2, 3, 4] * [-4,-3, 2, 1] = -4 - 6 + 6 + 4 = 0.
[1, 2, 3, 4] * [-2, 1,-4, 3] = -2 + 2 - 12 + 12 = 0.
[1, 2, 3, 4] * [-4, 3,-2, 1] = -4 + 6 - 6 + 4 = 0.
[1, 2, 3, 4] * [-2, 1, 4,-3] = -2 + 2 + 12 - 12 = 0.
[1, 2, 3, 4] * [-3, 4, 1,-2] = -3 + 8 + 3 - 8 = 0.
[1, 2, 3, 4] * [ 4, 1, 2,-3] = 4 + 2 + 6 - 12 = 0.
[1, 2, 3, 4] * [ 2, 3,-4, 1] = 2 + 6 - 12 + 4 = 0.
[1, 2, 3, 4] * [ 3, 4,-1,-2] = 3 + 8 - 3 - 8 = 0.
[1, 2, 3, 4] * [ 4, 3,-2,-1] = 4 + 6 - 6 - 4 = 0.
[1, 2, 3, 4] * [ 2,-1, 4,-3] = 2 - 2 + 12 - 12 = 0.
[1, 2, 3, 4] * [ 4,-3, 2,-1] = 4 - 6 + 6 - 4 = 0.
[1, 2, 3, 4] * [ 2,-1,-4, 3] = 2 - 2 - 12 + 12 = 0.
[1, 2, 3, 4] * [ 3,-4,-1, 2] = 3 - 8 - 3 + 8 = 0.
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