import java.util.Scanner;

public class Sum {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int num = sc.nextInt();
        if(num < 0){
            num = Math.abs(num);
            num = ((num*(num+1)/2)-1)*-1;
            System.out.println(num);
        }else if(num > 0 && num < 10000){
            num = num*(num+1)/2;
            System.out.println(num);
        }else if(num == 0){
            num = 1;
            System.out.println(num);
        }
        System.out.flush();
    }
}

Do not use scanner classes. I got WA for a program. Use the bufferedreader and then parse it.