23 без ограничений (вперед)

a = [0] * 10
a[2] = 1
for i in range(2, 10):
    if i + 1 <= 9:
        a[i + 1] += a[i]
    if i + 2 <= 9:
        a[i + 2] += a[i]
    if i + i - 1 <= 9:
        a[i + i - 1] += a[i]
print(a[9])