Now let's tackle adding two negatives:

    1 0 1 1 0 1 1 1     -73
  + 1 1 1 0 1 1 1 1     -17
  -----------------    ----
  1 1 0 1 0 0 1 1 0     -90

Again, if there is a final carry out, it gets thrown away. Everything works out fine and we get a negative answer. Remember that this will be true if our answer does not exceed -128.