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 Go (1.6) (judge verdict: Accepted)

package main

import (
	"fmt"
)

// main is ...
func main() {
	var a, b int
	fmt.Scan(&a, &b)

	isValid := isValid(a) && isValid(b)
	if isValid {
		fmt.Print(a * b)
		return

	}

	fmt.Print(-1)
}

func isValid(a int) bool {
	return 0 <= a && a < 10
}