# in: string
# out: boolen
def is_valid(num):
    digits = [int(n) for n in num]
    ttl = 0
    parity = len(digits) % 2
    for key, value in enumerate(digits):
        if key % 2 == parity:
            value *= 2
            if value > 9:
                value -= 9
        ttl += value
    rv = total % 10 == 0
    return rv