import java.util.*;
	class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        if(a<=10&&b<=10){
            System.out.println(-1);
        } else {
            System.out.println(a * b);
        }
    } 
}