#include <iostream>
#include <string>
#include <utility>
#include <algorithm>
#include <map>
#include <set>
#include <vector>
#include <cmath>
#include <cstdlib>
#include <queue>
#include <stack>
#include <iomanip>

using namespace std;

#define REP(i, n) for(ll i = 0;i < n;i++)
#define REPR(i, n) for(ll i = n;i >= 0;i--)
#define FOR(i, m, n) for(ll i = m;i < n;i++)
#define FORR(i, m, n) for(ll i = m;i >= n;i--)
#define REPO(i, n) for(ll i = 1;i <= n;i++)
#define ll long long
#define INF (ll)1 << 60
#define MINF (-1 * INF)
#define ALL(n) n.begin(),n.end()
#define MOD 1000000007
#define P pair<ll, ll>


int main() {
	ll a, b;
	cin >> a >> b;
	ll c = abs(a - b);
	if ((a + c) % b == 0 and (b + c) % a == 0 and (b + a) % c == 0) {
		cout << c << endl;
		return 0;
	}
	REP(i, 1000000) {
		c = (a + b) * i;
		if ((a + c) % b == 0 and (b + c) % a == 0 and (b + a) % c == 0) {
			cout << c << endl;
			return 0;
		}
	}
	cout << -1 << endl;
}