We are using Python 3 for this program.

American credit card numbers follow a standardized system. All credit card numbers have 16 digits, and the first digit identifies the card brand: Visa cards start with a 4, MasterCard cards with a 5, and Discover with a 6. Write a program that prompts for a credit card number (all mashed together, no spaces/hyphens) and tells the user one of the following:

it is not a valid card number (it is not 16 digits long);

it is a Visa, MasterCard, or Discover;

its brand cannot be determined (it doesn't starting with 4/5/6).