#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>

vector<ll> list;
int main() {
	ll a, b;
	cin >> a >> b;
	ll d = a + b;
	for (int i = 1; i <= sqrt(d); i++)
	{
		if (d % i == 0) {
			list.push_back(i);
			list.push_back(d / i);
		}
	}
	REP(i, list.size()) {
		ll c = list[i];
		if ((a + c) % b == 0 and (b + c) % a == 0 and (b + a) % c == 0) {
			cout << c << endl;
			return 0;
		}
	}

	cout << -1 << endl;
}