1.23a

import java.util.Scanner;

public class z1_23a {

    public static void main(String[] args) {
        Scanner read = new Scanner(System.in);
        int n;

        while (true){
            System.out.println("");
            System.out.printf("Podaj ilosc wyrazen ciagu [ > ] ");
            n = read.nextInt();

            if(n <= 0){
                System.out.println("");
                System.out.println("Podaj liczbe wieksza od 0");
                continue;
            } else {
                break;
            }
        }

        int suma = 0;
        int var = -2;
        for(int i=1; i<=n; i++){
            suma += var;
            if(var < 0){
                var*=(-1);
                var+=3;
            } else {
                var+=3;
                var*=(-1);
            }
        }

        System.out.println("");
        System.out.println("Suma = " + suma);

        read.close();
    }

}