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Credit Card Validator

You need to verify if the given credit card number is valid. For that you need to use the Luhn test.

Here is the Luhn formula:

  1. Reverse the number.
  2. Multiple every second digit by 2.
  3. Subtract 9 from all numbers higher than 9.
  4. Add all the digits together.
  5. Modulo 10 of that sum should be equal to 0.

Task:

Given a credit card number, validate that it is valid using the Luhn test. Also, all valid cards must have exactly 16 digits.

Input Format:

A string containing the credit card number you need to verify.

Output Format:

A string: 'valid' in case the input is a valid credit card number (passes the Luhn test and is 16 digits long), or 'not valid', if it's not.

Sample Input:

4091131560563988

Sample Output:

valid

Explanation:

Let's run the Luhn test for our input:

Reverse: 8893650651311904
Multiplying the even positions by 2: 8 16 9 6 6 10 0 12 5 2 3 2 1 18 0 8
Subtract 9 from >9: 8 7 9 6 6 1 0 3 5 2 3 2 1 9 0 8
The sum: 70
70 Modulo 10 is 0.

The input passed the Luhn test and contains 16 digits, making it a valid credit card number.