import java.util.Scanner;
 
class Sample {
  public static void main(String args[]) {
    Scanner scanner = new Scanner(System.in);
    String input = scanner.nextLine();
    int num = Integer.parseInt(input);

    for(int i = 1; i <= num; i++){
        num += i;
    }
    System.out.println(num);
    scanner.close();
  }
}