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;

        if (a == 0) {
            b = 1;
        } else if (a > 0) {
            b = ((1 + a) * a) / 2;
        } else if (a < 0) {
            for (int i = 1; i <= a; i--) {
                b += i;
            }
        }

        PrintWriter pw = new PrintWriter(new File("output.txt"));
        pw.print(b);
        pw.close();
    }
}