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