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 > 9 || B > 9) {
			System.out.println(-1);
		} else {
			System.out.println(A * B);
		}
	}
}
