$1$ Write a Java program given the following specification and

provide comments which explain how your algorithm works.

Problem Statement

The task is to take in a credit card number, and find out if it is a

valid credit card number or not. Credit card numbers follow an

algorithm called Luhn's algorithm.

The formula verifies a number against its check digit, which is

the last digit. This number must pass the following test:

From the rightmost digit, which is the check digit, and moving

left, double the value of every second digit. If the result of this

doubling operation is greater than $9($e$.$g$.,82=16),$ then add

the digits together $($e$.$g$.,16$:$1+6=7,18$:$1+8=9).$

Take the sum of all the digits.

If the total modulo $10$ is equal to $0($if the total ends in zero$)$

then the number is valid according to the Luhn formula; else it is

not valid.

Assume an example of an account number $"7992739871"$ that

will have a check digit added, making it of the form

$7992739871$x$.$ The sum of all the digits, processed as per steps

$1$ and $2,$ is $67+x.$ Thus $x$ must be $3$ to bring the total to be

modulo $10=0.$ If $x$ is not $3,$ then this is not a valid credit card

number.

Input Format

An $n-$digit credit card number, where the last digit is the check

digit.

Output Format

Output "VALID" if it is a valid credit card number and "INVALID"

if it is not.

Constraints

$4n30$

Sample Input

$4539682995824395$

Sample Output

VALID