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