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 < 15) {
//                    sum += i;
//                    continue;
//                }
                if (sign == 0) {
                    sum += i;
                } else {
                    sum -= i;
                }
            }
        }
        System.out.println(sum);


    }
}