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