0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,
0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,
20 is even.
19 is odd.
18 is even.
17 is odd.
16 is even.
15 is odd.
14 is even.
13 is odd.
12 is even.
11 is odd.
10 is even.
9 is odd.
8 is even.
7 is odd.
6 is even.
5 is odd.
4 is even.
3 is odd.
2 is even.
1 is odd.