import java.util.*;
import java.lang.*;
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>9 || B>9 ){
			System.out.println("-1");
		}else{
			System.out.println(A*B);
		}
	}
}
