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: Accepted)

#include<bits/stdc++.h>

#define F first
#define S second
#define ll long long
#define int long long
#define pb push_back
#define ld long double

using namespace std ; 

const int N = 1001 * 1001  , MOD = 1e9 + 7 ; 

int32_t main()
{
	ios::sync_with_stdio(0) ;cin.tie(0) ; 
	int x , y ; 
	cin >> x >> y ; 
	if(x%y==0) return cout << -1 << endl , 0 ; 
	cout << x << endl ; 
}