import java.util.Scanner;

public class Main {

	@SuppressWarnings("resource")
	public static void main(String args[]) {
		Scanner scanner = new Scanner(System.in);
		int n=scanner.nextInt();
		long max=Integer.MIN_VALUE;
		for(int i=0;i<=n;i++) {
			int j=n-i;
			int ut=i+1;
			long tmp=i+(long)j*ut;
			max=Math.max(max, tmp);
		}
		System.out.println(max);
	}
}