HDU1001

import java.util.Scanner;

/**
 * Created by mac on 2018/11/30.
 */
public class Main {
    public static void main(String[]args) {
        Scanner input = new Scanner(System.in);
        while (input.hasNextInt()) {
            int sum = input.nextInt();
            //求前n项和（1+n）* n /2;
            //在java计算中会考虑到／2为小数后需因int类型强制转化成整数
            if ((sum % 2) == 0)
                System.out.println(sum / 2 *(sum + 1));
            else
                System.out.println((sum + 1) / 2 * sum);
            System.out.println();
        }
        input.close();
    }
}