import sys

cardnumb_input = input('Enter your credit card number: ')


def list_tweak():
    try:
        cardnumb = [int(i) for i in cardnumb_input]
        cardnumb[0::2] = [x * 2 for x in cardnumb[0::2]]
        cardnumb = [str(i) for i in cardnumb]
        cardnumb = list(''.join(cardnumb))
        cardnumb = [int(i) for i in cardnumb]
        return cardnumb

    except ValueError:
        print('Please enter only number in format: xxxxxxxxxxxxxxxx')
        sys.exit(1)


def valid_or_not():
    total = sum(list_tweak())
    if total % 10 != 0:
        print('It seems that this credit card number is not valid.')
    else:
        print('This credit card number is valid. ')


valid_or_not()