def calculate_check_digit(number):
    """
    Given a number without the check-digit, calculate
    the check digit and return it.

    See: https://www.gs1.org/services/how-calculate-check-digit-manually
    """
    # Step 2: Multiply alternate position by 3 and 1
    # and get the sum
    step_2 = sum(
        [int(n) * 3 for n in number[::-1][::2]]
    ) + sum(
        [int(n) for n in number[::-1][1::2]]
    )

    # Subtract the sum from nearest equal or higher multiple of ten
    return str(10 - (step_2 % 10))