#include<iostream>
#include<string>
#include<cstdio>
#include<vector>
#include<cmath>
#include<algorithm>
#include<functional>
#include<iomanip>
#include<queue>
#include<ciso646>
#include<random>
#include<map>
#include<set>
#include<bitset>
#include<stack>
#include<unordered_map>
#include<utility>
#include<cassert>
using namespace std;

typedef long long ll;

typedef unsigned long long ul;
typedef unsigned int ui;
const ll mod = 998244353;
const ll INF = mod * mod;
typedef pair<int, int> P;
#define stop char nyaa;cin>>nyaa;
#define rep(i,n) for(int i=0;i<n;i++)
#define per(i,n) for(int i=n-1;i>=0;i--)
#define Rep(i,sta,n) for(int i=sta;i<n;i++)
#define rep1(i,n) for(int i=1;i<=n;i++)
#define per1(i,n) for(int i=n;i>=1;i--)
#define Rep1(i,sta,n) for(int i=sta;i<=n;i++)
typedef pair<ll, ll> LP;
typedef vector<int> vec;
typedef vector<string> svec;
typedef long double ld;
typedef pair<ld, ld> LDP;
const ld eps = 1e-8;

ll gcd(ll a, ll b) {
	if (a < b)swap(a, b);
	while (b) {
		ll r = a % b; a = b; b = r;
	}
	return a;
}
void solve() {
	ll a, b; cin >> a >> b;
	if (b%a == 0) {
		cout << -1 << endl; return;
	}
	ll d = (b + a - 1) / a;
	ll num = d * a;
		ll le = num / a + num % a;
		ll ri = num / b + (num%b / a) + (num%b%a);
		if (le < ri) {
			cout << num << endl; return;
		}
		else {
			cout << -1 << endl;
		}
}

signed main() {
	ios::sync_with_stdio(false);
	cin.tie(0);

	//cout << fixed << setprecision(10);
	//init();
	solve();
	//stop
	return 0;
}

