import java.util.*;
import java.lang.*;
import java.io.*;

class Main{
    public static int sum(int n, int acc) {
        if(n <= 0){return acc;}
        else{return sum(n-1, n+acc);}
    }
    public static void main (String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());
        System.out.println(sum(n, 0));
    }
}