#define _CRT_SECURE_NO_WARNINGS
#include "bits/stdc++.h"
using namespace std;
#define rep(i, n) for(int i=0; i<(n); ++i)
#define FOR(i, m, n) for(int i=(m); i<(n); ++i)
#define sz(x) ((int)(x).size())
#define all(x) (x).begin(),(x).end()
#define mp make_pair
#define pb push_back
#define Cout(x) cout << (x) << endl
#define dump(x) cerr << #x << " = " << (x) << endl;
typedef long long LL;
typedef vector<int> VI;
typedef vector<long long> VL;
typedef vector<string> VS;
typedef vector<vector<int>> VVI;
typedef pair<int, int> PII;
const int inf = (int)1e9;
const long long mod = (long long)1e9 + 7;
const double pi = acos(-1.0);

//約数列挙
vector<long long> DivAll(long long n) {
	vector<long long> ret;
	long long i = 1;
	for (; i * i < n; ++i)
		if (n % i == 0)ret.push_back(i);
	bool flag = i * i == n;
	if (flag)ret.push_back(i);
	for (i = (int)ret.size() - 1 - flag; i >= 0; --i)
		ret.push_back(n / ret[i]);
	return ret;
}

bool re(int a, int b, int c) {
	return (a != b && b != c && a != c) && ((a + b) % c == 0) && ((a + c) % b == 0) && ((b + c) % a == 0);
}

int main() {
	LL a, b; cin >> a >> b;
	VL div = DivAll(a + b);

	int ans = -1;
	for (auto n : div) {
		if (re(a, b, n)) {
			ans = n;
			break;
		}
	}
	Cout(ans);
}