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