import java.util.Scanner;

public class Task15 {

    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);

        System.out.print("Enter number: ");
        int num = scan.nextInt();
        int i=0,sum=0;
        do{
            if(num>0){
                i++;
                sum+=i;
            }else{
                i--;
                sum+=i;
            }
        }while(i<num || i>num);
        System.out.println(sum);
    }

}