import java.util.Scanner;

public class Zadanie2a {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Prosze podac liczbe N: ");
        int N = sc.nextInt();
        System.out.println("Suma ciagu f(N) = " +sum(N));
    }

    static int sum(int a){
        int[] tab = new int[a+1];
        tab[1] = 1;
        tab[2] = 3;
        for(int i = 3; i<tab.length; i++){
            tab[i] = tab[i-1]+i;
        }
        return tab[tab.length-1];
    }
}