import java.io.PrintStream;
import java.util.Scanner;

public class _1068 {


    PrintStream         out     ;
    Scanner             sc;
    public _1068()
    {
        try {
            sc       = new Scanner(System.in);
            out     = System.out;
        } catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

    protected void run() throws Exception {
        int n = sc.nextInt();
        int nb = Math.abs(n);
        if(n<0)
        nb+=2;
        out.println(((nb)*(n+1))/2);

        sc.close();
        out.close();
    }

    public static void main(String args[]) throws Exception {
        new _1068().run();
    }

}