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<cstdio>
#include<cstring>
#include<iostream>
#include<string>
#include<map>
#include<vector>
#include<queue>
#include<set>
#include<stack>
#include<sstream>
#include<cmath>
#include<algorithm>
using namespace std;
typedef long long ll;
const int N=200000+10;
ll n,m;
ll gcd(ll x,ll y){
    if(x%y==0) return y;
    return gcd(y,x%y);
}
int main() {
    while(~scanf("%lld%lld",&n,&m)){
        if(n%m==0){
            printf("-1\n");
        }
        else {
            ll x=n/(gcd(n,m));
            printf("%lld\n",x==1?n:x*x);
        }
    }

    return 0;
}