import java.util.*;

public class Main {
	
	public static void main(String[] args) {

		// argumentの取得
		Scanner sc = new Scanner(System.in);

		int x = sc.nextInt();
	    int y = sc.nextInt();
      
		// 戻り値の返却
        if ( x < 10 && y < 10) {
          System.out.println(x * y);
        } 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