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.

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.

Constraints

• $1 \leq A \leq 20$
• $1 \leq B \leq 20$
• All values in input are integers.

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

见题面 Input 部分

见题面 Output 部分