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: Wrong Answer)

#include <bits/stdc++.h>
using namespace std;
#define REP(i,n) for(int i=0;i<n;i++)
typedef priority_queue <int> pi;
typedef priority_queue <int, vector<int>, greater<int>> pg;
typedef unsigned long long int ulli;

int main() {
    ulli X, Y;
    cin >> X >> Y;
    if(Y % X == 0 || X % Y == 0){
        cout << "-1" << endl;
    }else{
        int k = 1;
        int ans = 0;
        while(1){
            ++k;
            ans = X * k;
            if(ans % Y != 0 || Y % ans != 0) break;
        }
        cout << ans;
    }
}