import java.math.BigInteger;
import java.util.Scanner;

public class Solution {
	static void sumOfGroup(int k) {
		BigInteger n = new BigInteger("" + k);
		n = n.multiply(n.subtract(BigInteger.ONE));
		n = n.divide(new BigInteger("" + 2));
        BigInteger bk = new BigInteger("" + k);
        BigInteger br = n.add(bk);
        br = br.multiply(br);
        br = br.subtract(n.multiply(n));
        System.out.println(br.toString());
    }

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int k = in.nextInt();
        sumOfGroup(k);
        in.close();
    }
}