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