|   a   |   b   |   c   |  sum  | carry |
|   0   |   0   |   0   |   0   |   0   |
|   0   |   0   |   1   |   1   |   0   |
|   0   |   1   |   0   |   1   |   0   |
|   0   |   1   |   1   |   0   |   1   |
|   1   |   0   |   0   |   1   |   0   |
|   1   |   0   |   1   |   0   |   1   |
|   1   |   1   |   0   |   0   |   1   |
|   1   |   1   |   1   |   1   |   1   |