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

public class MainTest {

    static long sumOfGroup(int k) {
    	
    	long sum = 0;
    	long fac = factorial(k);
    	long prev = fac - k;
    	
    	for(long i = prev*2-1+2; i <= fac*2-1; i+=2) {
    		if (Math.ceil(i/2.0) > prev)
        		sum+=i;
        }
    	return sum;
    }
    
    public static long factorial(int n) {
        long result = 0;
        for (long i = 1; i <= n; i++) {
            result = result + i;
        }
        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();
    }
}