import java.util.*;
public class JavaApp {

    public static void main(String[] args) {
             int i=0;

                Scanner scan = new Scanner(System.in);
        int n=scan.nextInt();

                if ((n>=1)&&(n<10000)){
                    i=n *(n + 1) / 2;
                    System.out.print(i);}
                else
                if ((n<=1)&&(n>-10000)){
                    i=-((-n) *(1 -n) / 2) + 1;
                    System.out.print(i);}
        };
    }

import java.util.Scanner;

public class tests {


    public static void main(String[] args) {
             int i=0;

                Scanner scan = new Scanner(System.in);
                try{
                int n=scan.nextInt();
                if ((n>=1)&&(n<=10000)){
                    i=n *(n + 1) / 2;
                    System.out.print(i);}
                else
                if ((n<=1)&&(n>=-10000)){
                    i=-((-n) *(1 -n) / 2) + 1;
                    System.out.print(i);}
                }
                finally
                {
                    scan.close();
                    }
        }
}