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