from cs50 import get_string


def invalid():
    print("INVALID")
    exit()


# check luhn alghorythm
def luhn_algo():
    nSum = 0
    isSecond = False
    # iterate over all card numbers from back
    for i in range(ndigits - 1, -1, -1):
        d = int(user_input[i])

        # check if number is every second
        if isSecond:
            d *= 2
        # check for double digit sums
        if d >= 10:
            nSum += int(d / 10)
            nSum += int(d % 10)
        else:
            nSum += d

        if isSecond:
            isSecond = False
        else:
            isSecond = True
    if nSum % 10 == 0:
        return True
    else:
        invalid()


# check validity of card by using luhn alghoritm
def card_check():
    beginning = int(user_input[0:2])
    if 39 < beginning < 50:
        if luhn_algo():
            print("VISA")
    elif beginning == 34 or beginning == 37:
        if luhn_algo():
            print("AMEX")
    elif 50 < beginning < 56:
        if luhn_algo():
            print("MASTERCARD")
    else:
        invalid()


# takes user input and check if the value is positive number
user_input = get_string("Enter credit card number:")
try:
    if int(user_input) < 1:
        invalid()
except:
    invalid()

# calculates the length of input and if correct, continues with card_check
ndigits = len(user_input)
if ndigits == 15 or ndigits == 13 or ndigits == 16:
    card_check()
else:
    invalid()