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>
#pragma GCC optimize("O3")
using namespace std;
#define ll long long 
#define pb push_back
//#define mp make_pair
#define fr first
#define sc second
#define N ((ll)(2e6+100))
#define MAX ((ll)(1e16+100))
#define ARRS ((ll)(3e5+100))
#define MOD ((ll)(1e9+7))
#define M1 ((ll)(1001783))
#define M2 ((ll)(1001941))
#define M3 ((ll)(1002073))
#define pb push_back


ll n,m;
ll a[ARRS];
string s;

int main(){
	cin>>n>>m;
	if(n%m)
		cout<<n;
	else cout<<-1;
}