import java.util.*;
import  java.io.*;

public class Main {

    public static void main(String[] args)throws IOException {
        Scanner sc = new Scanner(new File("input.txt"));
        int a = sc.nextInt();
        sc.close();
        int b=0;
        int i;
        if(a==0) {
            b = 1;
        }else if(a>0) {
            b = ((1+(a))*(a))/2;
            }else if(a<0) {
            for (i = 1; i <= a; i--) {
                b += i;
            }
        }
        PrintWriter pw = new PrintWriter(new File("output.txt"));
        pw.print(b);
        pw.close();

    }
}