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