import java.util.*;
public static void main(String args[]){
  Scanner scn = new Scanner(System.in);
  int a = scn.nextInt();
  int b = scn.nextInt();
  if((a < 10) && (b < 10)){
    System.out.println(a*b);
  }else{
    System.out.println(-1);
  }
}
