import java.util.*;

public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int A = sc.nextInt();
    int B = sc.nextInt();
    sc.close();
    if (A>=10||B>=10) System.out.println(-1);
    else System.out.println(A*B);
    return;
  }
}
