Sentimental/Credit does not check if Luhns algorithm is implemented

[mcchots]

This code passes all checks in check50 and scores 100% with submit50

from cs50 import get_string

number = get_string("Number: ")

if len(number) == 15 and number[0] == "3":
    print("AMEX")
elif len(number) == 16 and number[0] == "4":
    print("VISA")
elif len(number) == 16 and number[0] == "5":
    print("MASTERCARD")
else:
    print("INVALID")