import java.util.Scanner;
public class Bil {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        int a = input.nextInt();
        int b = numbers (Math.abs(a));
        if (a >= 0 )System.out.println(b);
        else {System.out.println(-b);}
    }


    public static int numbers( int a){
        int sum = 0;
        if (a > 1){
            for (int i = 2; i <= a; i++)
            sum +=i;
            }
//        else if (a == 0) sum = 0;
        else{
            sum = 1;
            }
        return sum;
    }
}