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>
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define sz(x) x.size()
#define all(x) x.begin(),x.end()
#define forn(i,n) for(int i=0;i<n;++i)
#define repn(i,n) for(int i=1;i<=n;++i)
#define TC(n) while(n--)
#define forb(i,n) for(int i=n-1;i>=0;--i)
#define repb(i,n) for(int i=n;i>=1;--i)

typedef long long LL;
typedef unsigned long long ULL;

using namespace std;
int x,y;
bool cek = false;
int main(){
	scanf("%d%d",&x,&y);
	if(!(x%y)){
		printf("-1\n");
	}else{
		printf("%d\n", x*2);
	}
	return 0;
}