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++ (GCC 9.2.1) (judge verdict: Accepted)

/*#pragma GCC optimize("Ofast")
#pragma GCC target("avx,avx2,fma")
#pragma GCC optimization("unroll-loops")
*/

#include <bits/stdc++.h>
using namespace std;
#define pi acos(-1)
#define IOS ios_base::sync_with_stdio(0); cin.tie(); cout.tie();
#define fi first
#define se second
#define pf push_front
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define debug(x) cerr<<#x<<": "<<(x)<<'\n'
typedef long double ld;
typedef long long ll;
typedef unsigned long long ull;
const ll mod = 1e9+7;
const ll inf = 0x3f3f3f3f;
const ll nax = 0;
ll a, b;

int main(){
	IOS
	cin >> a >> b;
	if(a%b==0){
		cout << -1 << '\n';
		return 0;
	}
	cout << a << '\n';
}