Problem:
Score : 100 points

Problem Statement

You are given positive integers X and Y.
If there exists a positive integer not greater than 10^{18} that is a multiple of X but not a multiple of Y, choose one such integer and print it.
If it does not exist, print -1.

Constraints

1 ≤ X,Y ≤ 10^9

X and Y are integers.

Input

Input is given from Standard Input in the following format:

X Y

Output

Print a positive integer not greater than 10^{18} that is a multiple of X but not a multiple of Y, or print -1 if it does not exist.

Sample Input 1

8 6

Sample Output 1

16

For example, 16 is a multiple of 8 but not a multiple of 6.

Sample Input 2

3 3

Sample Output 2

-1

A multiple of 3 is a multiple of 3.

Solution in C++14 (GCC 5.4.1) (judge verdict: Wrong Answer)

#include<cstdio>
#include<iostream>
#include<algorithm>
#include<cstring>
#include<cmath>
using namespace std;

int main(){
	int n,m;
	int s;
	int d;
	d = -1;
	scanf("%d %d",&n,&m);
	if(n > m)
		s = n % m;
	else
		s = m % n;
	if(s == 0)
		printf("%d\n",d);
	else
		for(int i = 2;i<1000;i++){
			s = i*n;
			if(s % m != 0&&m % s != 0){
				printf("%d\n",s);
			}
			break;
		}
	return 0;
}