Problem:
Score : 100 points

Problem Statement

Having learned the multiplication table, Takahashi can multiply two integers between 1 and 9 (inclusive) together. He cannot do any other calculation.

Given are two integers A and B.

If Takahashi can calculate A \times B, print the result; if he cannot, print -1 instead.

Constraints

1 \leq A \leq 20

1 \leq B \leq 20

All values in input are integers.

Input

Input is given from Standard Input in the following format:

A B

Output

If Takahashi can calculate A \times B, print the result; if he cannot, print -1.

Sample Input 1

2 5

Sample Output 1

10

2 \times 5 = 10.

Sample Input 2

5 10

Sample Output 2

-1

5\times 10 = 50, but Takahashi cannot do this calculation, so print -1 instead.

Sample Input 3

9 9

Sample Output 3

81

Solution in OCaml (4.10.0) (judge verdict: Accepted)

let () = Scanf.scanf "%d %d" @@ fun a b ->
  let array = Array.make 100 false in
  for i=1 to 9 do
    for j=1 to 9 do
      array.(i*j) <- true
    done
  done;
  Printf.printf "%d\n" @@
  if a <= 9 && b <= 9
  then a*b
  else -1