import java.util.Scanner;

public class A000480 {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int in = sc.nextInt();
		sc.close();
		for(int i=1,r=0;i<=in;i++) {
			r += i;
			if(i==in)System.out.println(r);
		}
	}

}