import java.util.*;

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(a * b);
		} else {
			System.out.println(-1);
		}
		
	}
}