// This program tells you what the last digit of a credit card should be.
// This is an implementation of the Luhn algorithm.

import java.util.*;

public class CreditCard {
    public static void main(String[] args) {
        Scanner console = new Scanner(System.in);
        System.out.println("This program tells you the last digit of a");
        System.out.println("credit card.");
        System.out.println();
        System.out.print("leading digits (all but last)? ");
        String number = console.next();
        System.out.println("The last digit should be " + lastDigitFor(number));
    }

    // This method takes a partial credit card number (all but the last
    // digit) and computes what the last digit should be, returning it
    // as an int.
    public static int lastDigitFor(String card) {
        int multiplier = 2;
        int sum = 0;
        for (int i = card.length() - 1; i >= 0; i--) {
            int digit = Character.getNumericValue(card.charAt(i));
            int n = digit * multiplier;
            sum = sum + n / 10 + n % 10;
            if (multiplier == 2) {
                multiplier = 1;
            } else {
                multiplier = 2;
            }
        }
        return (10 - sum % 10) % 10;
    }
}