+----+----+----+----+----+----+
| | - | | + | |= 3 |
+----+----+----+----+----+----+
| + | | x | | x | |
+----+----+----+----+----+----+
| | + | | - | |= 9 |
+----+----+----+----+----+----+
| - | | + | | - | |
+----+----+----+----+----+----+
| | - | | x | |= 2 |
+----+----+----+----+----+----+
|= 2 | |= 7 | |= 3 | |
+----+----+----+----+----+----+