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)

// Contest.cpp : コンソール アプリケーションのエントリ ポイントを定義します。
//

#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <list>
#include <set>
#include <numeric>
#include <map>
#include <math.h>
#include <iomanip>
#include <stack>

using namespace std;


int main() {
	int x, y;
	cin >> x >> y;
	if (x%y == 0) {
		cout << -1;
		return 0;
	}
	else {
		for (long long int i = x; i < 100000000000000000; i += x) {
			if (i%y == 0)continue;
			else {
				cout << i;
				return 0;
			}
		}
	}
	return 0;
}