level:
1 [x]
2 [x]
3 [x]
4 [x]
5 [x]
6 [x]
7 [x]
8 [x]
9 [x]
10 [x]
11 [x]
12 [x]
13 [x]
14 [x]
15 [x]
16 [x]
17 [x]
18 []
19 []
20 []
21 []
22 []
23 []
24 []
25 []
26 []
27 []
28 []
29 []
30 []