package yukicoder;

import java.util.Scanner;

public class No480 {
	public static void main(String[] args) {
		//Scannerを使用し、入力
		Scanner sr = new Scanner(System.in);

		//iを入力
		int i = sr.nextInt();

		//ans 初期化
		int ans = 0;

		for (; i < 100; i--) {
			ans = ans + i;

			if (i == 0) {
				break;
			}
		}
		System.out.println(ans);
	}
}