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