import java.io.*;
import java.util.*;
import java.text.*;
import java.math.*;
import java.util.regex.*;

public class Solution {

    static BigInteger sumOfGroup(int k) {
        // Return the sum of the elements of the k'th group.
        long startIndex = (long)(k)*(k-1)/2;
        long endIndex = startIndex+k;
        //System.out.println(startIndex+":"+endIndex+":"+(endIndex*endIndex)+":"+(startIndex*startIndex));
        BigInteger largerSum = new BigInteger(endIndex+"").multiply(new BigInteger(endIndex+""));
        BigInteger smallerSum = new BigInteger(startIndex+"").multiply(new BigInteger(startIndex+""));   
        //System.out.println(largerSum+":"+smallerSum);
        return largerSum.subtract(smallerSum);//.longValue();
    }

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