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