Lucky key is 8


8,it took 1 times
3,4,5,1,6,7,8,it took 7 times
5,2,1,3,6,7,8,it took 7 times
2,3,5,8,it took 4 times
7,3,8,it took 3 times
1,3,2,5,8,it took 5 times
3,2,7,1,4,8,it took 6 times
7,4,3,1,6,5,2,8,it took 8 times
5,2,4,1,3,6,8,it took 7 times
3,2,4,5,1,8,it took 6 times
1,6,5,4,8,it took 5 times
6,1,2,3,7,8,it took 6 times
3,4,1,5,7,6,8,it took 7 times
2,8,it took 2 times
8,it took 1 times
3,6,4,5,1,7,8,it took 7 times
4,7,6,8,it took 4 times
7,2,3,1,4,5,6,8,it took 8 times
8,it took 1 times
1,4,6,5,8,it took 5 times

The average is 5