#include <bits/stdc++.h>
//#include <atcoder/all>
using namespace std;
//using namespace atcoder;
//using mint = modint1000000007;
//const int mod = 1000000007;
//using mint = modint998244353;
//const int mod = 998244353;
//const int INF = 1e9;
//const long long LINF = 1e18;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep2(i,l,r)for(int i=(l);i<(r);++i)
#define rrep(i, n) for (int i = (n-1); i >= 0; --i)
#define rrep2(i,l,r)for(int i=(r-1);i>=(l);--i)
#define all(x) (x).begin(),(x).end()
#define allR(x) (x).rbegin(),(x).rend()
#define endl "\n"
#define P pair<int,int>
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }
vector<long long> DivisorEnumeration(long long n) {
	vector<long long> x;
	for (long long i = 1; i*i <= n; i++) {
		if (0 == n % i) {
			x.push_back(i);
			if (n != i * i) x.push_back(n / i);
		}
	}
	sort(all(x));
	return x;
}
int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int a, b; cin >> a >> b;
	auto v = DivisorEnumeration(a + b);
	rep(i, v.size()) {
		int c = v[i];
		if (c == a)continue;
		if (c == b)continue;
		if (0 != (b + c) % a)continue;
		if (0 != (a + c) % b)continue;
		if (0 != (a + b) % c)continue;
		cout << c << endl;
		return 0;
	}
	cout << -1 << endl;
	return 0;
}