Atcoder apc001 A - Two Integers

#include <cstdio>
#include <cstdlib>
#include <iostream>
#include <cstring>
#include <string>
#include <algorithm>
#include <cmath>
#include <vector>
#include <queue>
#include <stack>
#include <deque>
#include <map>
#include <unordered_map>
#include <set>
#include <unordered_set>
typedef long long int ll;
using namespace std;
#define INF 1234567890
ll x, y;

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	cin >> x >> y;
	ll ans;
	if (x%y == 0) {
		cout << -1;
	}
	else {
		cout << x;
	}
}