odd even
+----|----+
| 1 | 2 |
+----+----|
| 3 | 4 |
+----+----|
| 5 | 6 |
+----+----|
| 7 | 8 |
+----+----|
| 9 | 10 |
+----+----|
| 11 | 12 |
+----+----|
| 13 | 14 |
+----+----|
| 15 | 16 |
+----+----|
| 17 | 18 |
+----+----|
| 19 | 20 |
+----+----|