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

public class Solution {

    static long sumOfGroup(long k) {
        // Return the sum of the elements of the k'th group.
        long n1=(k*(k-1))/2;
        long n2=(k*(k+1))/2;
    long s1=(n1*(2+(n1-1)*2))/2;
    long s2=(n2*(2+(n2-1)*2))/2;
    return s2-s1;
        
         

    }

    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();
    }
}