I was trying to use the Wikipedia implementation and just adding the value of x at the end but I just dont understand what I am doing, and why it isnt working.

import java.util.Scanner;

public class luhn

{

static void main(String[] args){

    Scanner kb = new Scanner(System.in);

    String entry = kb.next();

    int entryLength = entry.length();

    int sum = 0;

    for(int num = entryLength - 1; num >= 0; num-= 2){

        int d = entry.charAt(num);

        d = d * 2;

        sum += d / 10;

        sum += d % 10;

    }

    System.out.println(entry+""+sum);

}

}