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