import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.util.StringTokenizer;


public class No1068 {
    public static void main(String[] args) throws Exception {
        BufferedReader rd=new BufferedReader(new InputStreamReader(System.in));
        PrintWriter wr=new PrintWriter(new OutputStreamWriter(System.out));
        StringTokenizer line =new StringTokenizer(rd.readLine());
        Integer n=Integer.parseInt(line.nextToken());

        if (n>0)
            wr.println((n*(n+1))/2);

            else
            {
                n=Math.abs(n);
                wr.println(1-(n*(n+1)/2));
            }
        wr.close();
        System.exit(0);



    }

}