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EXAMPLE
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a(2) = 7: {1,3}, {1,8}, {2,7}, {3,6}, {4,5}, {6,10}, {7,9}.
a(3) = 20: {1,2,6}, {1,3,5}, {1,4,11}, {1,5,10}, {1,6,9}, {1,7,8}, {2,3,4}, {2,3,11}, {2,4,10}, {2,5,9}, {2,6,8}, {3,4,9}, {3,5,8}, {3,6,7}, {4,5,7}, {4,10,11}, {5,9,11}, {6,8,11}, {6,9,10}, {7,8,10}.
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