9 + 9 =  - 18, 6 + 6 =  - 12, 3 + 3 = - 6, 4 + 4 = - 8, 8 + 8 = - 16, 2 + 2 = - 4, 7 + 7 = - 14, 5 + 5 = - 10, 10 + 10 = - 20, 0 + 0 = - 0,

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