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: Time Limit Exceeded)

#include <bits/stdc++.h>
using namespace std;

int main() {
    long long X, Y;
    cin >> X >> Y;

    long long ans=1;

    //ans%X==0 && ans%Y!=0 ならans
    // みたさないなら-1を出力
    while(true)
    {
        if(X==Y)
        {
            cout << "-1" << endl;
            break;
        }

        if(ans % X == 0 && ans % Y != 0 )
        {
            cout << ans << endl;
            break;
        }

        ans++;

        if(ans == 1000000000000000000)
        {
            break;
        }

    }

}