Number the treads in regular order upwards, 1 to 8. Then proceed as
follows: 1 (step back to floor), 1, 2, 3 (2), 3, 4, 5 (4), 5, 6, 7 (6),
7, 8, landing (8), landing. The steps in brackets are taken in a
backward direction. It will thus be seen that by returning to the floor
after the first step, and then always going three steps forward for one
step backward, we perform the required feat in nineteen steps.