package com.company;

import java.util.HashMap;
import java.util.Map;
import java.util.Random;

public class prog {
    public static void main(final String[] args) {
        int sum=15;
        java.util.Scanner scan = new java.util.Scanner(System.in);
        int n = scan.nextInt();
        if(n>0 && n<=Math.pow(10,6)){

        int sign;
        for(int i=1;i<=n;i++){
           sign= new Random().nextInt(2);
            if(sum<15){
                sum+=i;
                continue;
            }
            if(sign==0){
                sum+=i;
            }
            else{
                sum-=i;
            }
        }}
        System.out.println(sum);

    }
}