def save_fill(C, J):
    """Fill question marks at beginning, up to one before the first digit."""
    first_digit = 0
    for c, j in zip(C, J):
        if c != '?' or j != '?':
            break
        first_digit += 1
    for i in range(first_digit-1):
        if C[i] == '?':
            C = list(C)
            C[i] = "0"
            C = "".join(C)
        if J[i] == '?':
            J = list(J)
            J[i] = "0"
            J = "".join(J)
    return C, J