import java.util.Scanner;
public class calcu{
    public static void main(String[] args){
        Scanner stdIn = new Scanner(System.in);
        int sum = 0;
        int i;
        int n = stdIn.nextInt();
        for(i=1; i>=n && n<=100; i++){
    	    sum += i;
        }
    System.out.println(sum);
    stdIn.close();
    }
}