import java.util.*;

public class TicketSales {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        int numberOfTickets = scanner.nextInt();
        int pricePerTicket = scanner.nextInt();

        if (numberOfTickets < 1) {
            System.out.println("Invalid input");
        } else if (pricePerTicket < 1) {
            System.out.println(-1);
        } else {
            int total = numberOfTickets * pricePerTicket;
            System.out.println(total);
        }

        Scanner.close();
    }
}