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>
using namespace std;
typedef pair<int,int> ii;
typedef vector<int>   vi;
typedef vector<double>   vd;
typedef vector<long long> vll;
typedef vector<pair<int,int> > vii;
#define ll long long
#define pb push_back
#define mp make_pair
#define ff first
#define ss second
#define pq priority_queue
#define mii map<int,int>
#define sf1(x) scanf("%d",&x)
#define sf2(x,y) scanf("%d %d",&x,&y)
#define sf3(x,y,z) scanf("%d %d %d",&x,&y,&z)
#define log2(x) (31 - __builtin_clz(x))
#define lcm(x,y) (x*y) / __gcd(x,y)
int inf = (1e9) + 7;
int mod = 998244353;

int main(){
	

	int x,y;
	sf2(x,y);

	if(!(x%y)){
		printf("-1\n");
	}
	else
		printf("%d\n",x);

}