import java.util.Scanner;

public class Main {

    public void solucion(int num) {

        int aux = 0;
        if (num <0) {
            for (int i = num; i <= 1; i++) {
                aux += i;
            }
        } else {
            for (int j = num; j >= 1; j--) {
                aux += j;
            }
        }
        System.out.println(aux);

    }

    public static void main(String[] args) {
        Main obj = new Main();
        Scanner leer = new Scanner(System.in);
        int numero = leer.nextInt();
        if (numero < 10000) {
            obj.solucion(numero);
        }


    }
}