def digits(n):
    if n < 10:
        return 1
    return 1 + digits(n // 10)

print(digits(1234))

def tail_sum(n, acc=0):
    if n == 0:
        return acc
    return tail_sum(n - 1, acc + n)

print(tail_sum(5))

cache = {}

def fib(n):
    if n in cache:
        return cache[n]
    if n <= 1:
        cache[n] = n
    else:
        cache[n] = fib(n - 1) + fib(n - 2)
    return cache[n]

print(fib(6))

def count_items(lst):
    count = 0
    for item in lst:
        if isinstance(item, list):
            count += count_items(item)
        else:
            count += 1
    return count

print(count_items([1, [2, 3], [4, [5]]]))