23 c обязательной

a = [0] * 16
a[1] = 1
for i in range(2, 9):
    a[i] += a[i - 1]
    if i - 3 >= 0:
        a[i] += a[i - 3]
for i in range(9, 16):
    a[i] += a[i - 1]
    if i - 3 >= 8:
        a[i] += a[i - 3]
print(a[16])