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