import java.util.Scanner;

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