import java.util.Scanner;

class Main {
	public static void main(String[] args) {
		try (Scanner sc = new Scanner(System.in)) {
			int a = sc.nextInt();
			int b = sc.nextInt();
			boolean bl = a < 10 && b < 10;
			if (bl) {
				System.out.println(a * b);
			} else {
				System.out.println(-1);
			}
		}
	}
}
/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