import java.util.*;
import java.lang.*;
import java.io.*;

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");
      	
      	
    }
}