package atcoder_144;
import java.util.Scanner;
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);
		}
	}
}
