task 13

def f(a, b):
    if a > b:
        return 0
    if a == b:
        return 1
    if a < b:
        return f(a+1, b) + f(a+2, b)


print(f(2, 9)*f(9, 18) - f(2, 9)*f(9, 14)*f(14, 18))