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