import java.util.Scanner;

public class Main {

	public static void main(String[] args) {


		Scanner sc = new Scanner(System.in);
		String a = sc.next();
		String b = sc.next();
		if(a.length() >1 || b.length() >1) {
			System.out.print(-1);
		}else {
			System.out.println(Integer.valueOf(a) * Integer.valueOf(b));
		}


	}
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz