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 C (GCC 5.4.1) (judge verdict: Accepted)

#include <stdio.h>
#include <assert.h>
#include <stdlib.h>
#include <string.h>

//#define NDEBUG

#ifdef NDEBUG
#define DEBUG_PRINT(fmt, ...) ((void)0)
#else
#define DEBUG_PRINT(fmt, ...) fprintf(stderr, fmt, ##__VA_ARGS__)
#endif
#define ASSERT_RANGE(min,x,max) assert(((x)>=(min)) && ((x)<=(max)))


#define N_MIN 1
#define N_MAX 20

int sort_int(int *a, int N) {// {{{
	int i;
	for (i = 0; i < N-1; i++) {
		int j;
		for (j = i + 1; j < N; j++) {
			if (a[i] > a[j]) {
				int tmp = a[i];
				a[i] = a[j];
				a[j] = tmp;
			}
		}
	}
	return 0;
}// }}}

int main(int argc, char **argv) {
	int A,B;
	fscanf(stdin,"%d%d",&A,&B);
	ASSERT_RANGE(N_MIN,A,N_MAX);
	ASSERT_RANGE(N_MIN,B,N_MAX);

	if ((A<10)&&(B<10)) {
		fprintf(stdout,"%d\n",A*B);
	} else {
		fprintf(stdout,"-1\n");
	}
	return 0;
}