X ( 2 , 9 , 3 , 10 ) X ( 4 , 18 , 5 , 17 ) X ( 6 , 19 , 1 , 20 ) X ( 7 , 16 , 8 , 17 ) X ( 8 , 1 , 9 , 2 ) X ( 10 , 3 , 11 , 4 ) X ( 12 , 6 , 13 , 5 ) X ( 13 , 20 , 14 , 21 ) X ( 15 , 22 , 16 , 7 ) X ( 18 , 11 , 19 , 12 ) X ( 21 , 14 , 22 , 15 ) {\displaystyle X(2,9,3,10)X(4,18,5,17)X(6,19,1,20)X(7,16,8,17)X(8,1,9,2)X(10,3,11,4)X(12,6,13,5)X(13,20,14,21)X(15,22,16,7)X(18,11,19,12)X(21,14,22,15)}