Split a char or an int :: Python 3

Question

I am doing a self-paced course, CS50 on EDX platform, the goal is to ask a user for a credit card number and then validate if it's a valid Amex, Master or Visa card.

I'm stuck trying split a number greater than 10 (formatted as a String) so that the result is for example "14" = "1" and "4" the idea is that both numbers will be appended to a list[].

I am trying to implement a credit card checksum check based on the algorithm invented by Hans Peter Luhn, I am stuck in the second part of the algorithm (written as a comment on the code) because as the algorithm specifies I have to split 14 into 1 4 and THEN do the sum.

Credit card for test: 378282246310005 So far i have this result: 14 + 4 + 4 + 8 + 6 + 0 + 0 = 36 //Bad and what i need to keep going is: 1 + 4 + 4 + 4 + 8 + 6 + 0 + 0 = 27 //Good

Here is my code:

def main():
    cCNumber = str(input("Ingrese el numero de la tarjeta de credito: "))
    if cCNumber[0] == "3" and (cCNumber[1] == "4" or cCNumber[1] == "7") :
            numbers = listInts(cCNumber)
            listSumador = checkSum(numbers)
            listSumadorStr = str(listSumador)
            print(listSumadorStr)
            print("Amex")

#Convert string to a list of Ints.
def listInts(stRnumber):
    listNumbers = []
    for i in range(0,len(stRnumber),1):
        listNumbers.append(int(stRnumber[i]))
    return listNumbers

#Implement the following algorithm:
    #1: Multiply every other digit by 2, starting with the number’s second-to-last digit, and then add those products' digits together.
    #2: Add the sum to the sum of the digits that weren’t multiplied by 2.
    #3: if the total’s last digit is 0 (or, put more formally, if the total modulo 10 is congruent to 0), the number is valid!
def checkSum(numbers):
    sumador = []
    for i in range(1,len(numbers),2):
        provisoryList = []
        provisoryList.append(int(numbers[i]) * 2)
        sumador.append(int(numbers[i]) * 2) 
    return sumador

if __name__ == "__main__":
    main()