import java.util.*;
public class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        
        if((a>0 && a<19) && (b<10 && b>0)){
            System.out.println(a*b);
        }else{
            System.out.println(-1);
        }
    }
}