def sum_of_squared_digits(n: int) -> int:
    total = 0
    while n > 0:
        digit = n % 10
        total += digit * digit
        n //= 10
    return total

def sequence_length(a0: int) -> int:
    seen = set()
    current = a0
    count = 0

    while True:
        count += 1
        if current in seen:
            break
        seen.add(current)
        current = sum_of_squared_digits(current)

    return count