import java.util.Scanner;

public class Q1 {
	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);
	}
}
