X ( 2 , 12 , 3 , 11 ) X ( 4 , 14 , 5 , 13 ) X ( 6 , 16 , 7 , 15 ) X ( 8 , 2 , 9 , 1 ) X ( 10 , 18 , 11 , 17 ) X ( 12 , 4 , 13 , 3 ) X ( 14 , 6 , 15 , 5 ) X ( 16 , 8 , 17 , 7 ) X ( 18 , 10 , 1 , 9 ) {\displaystyle X(2,12,3,11)X(4,14,5,13)X(6,16,7,15)X(8,2,9,1)X(10,18,11,17)X(12,4,13,3)X(14,6,15,5)X(16,8,17,7)X(18,10,1,9)}