Design alternative 1:
+---+---+---+---+ +------+
| | | | | | * * |
| 1 | 2 | 0 | 1 | | |
| | | | | | . |
+---+---+---+---+ +------+
| | | | | | |
| | | | | | |
| | | | | | |
+---+---+---+---+ +------+
Design alternative 2:
+---+---+---+---+ +-----+-----+-----+
| 1 | 2 | 0 | 1 | | + 2 | = 1 | - 1 |
+---+---+---+---+ +-----+-----+-----+
| | | | | | | | |
+---+---+---+---+ +-----+-----+-----+