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)

#include <bits/stdc++.h>

using namespace std;

#define uint unsigned int
#define llong long long int
#define ullong unsigned long long int
#define rep(i, n) for (int i = 0; i < n; ++i)

const static long long int MOD = 1000000000 + 7;
const static int dy[] = {0, 1, 0, -1};
const static int dx[] = {1, 0, -1, 0};

int main (int argc, char *argv[]) {
    cin.tie(0);
    ios::sync_with_stdio(false);

    llong x, y;
    cin >> x >> y;
    if (x % y == 0) cout << -1 << endl;
    else cout << x << endl;

    return 0;
}