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

public class Solution {

        static long sumOfGroup(long k) {
            if (k == 1) return 1;
            long first = k * (k - 1) / 2;
            long last = k * (k + 1) / 2;
            long firstSum = first * (first + 1) / 2;
            long lastSum = last * (last + 1) / 2;
            long result = (lastSum - firstSum) * 2 - k;
            return result;
        }

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