#include <bits/stdc++.h>


using namespace std;
typedef long long ll;
#define REP(i,n) for(int i=0; i<int(n); i++)
#define FOR(i,m,n) for(int i=int(m); i<int(n); i++)
#define ALL(obj) (obj).begin(),(obj).end()
#define VI vector<int>
#define VLL vector<long long>
#define VVI vector<vector<int>>
#define VVLL vector<vector<long long>>
#define VC vector<char>
#define VS vector<string>
#define VVC vector<vector<char>>
#define VB vector<bool>
#define VVB vector<vector<bool>>
#define fore(i,a) for(auto &i:a)

typedef pair <int, int> P;
template<class T> bool chmax(T &a, T b) { if (a < b) { a = b; return true; } return false; }
template<class T> bool chmin(T &a, T b) { if (a > b) { a = b; return true; } return false; }
const int  INF = 1 << 29;
const ll INFL = 1LL << 60;
const ll mod = 1000000007;





int main() {
	
	ll a, b;
	cin >> a >> b;
	ll c = a + b;

	ll ans = INFL;

	for (ll i = 1; i*i <= c; i++) {
		if (c%i == 0) {
			if ((a + i) % b == 0 && (b + i) % a == 0) {
				if (a != i && b != i)	ans = min(ans, i);
			}
			if ((a + (c / i)) % b == 0 && (b + (c / i)) % a == 0) {
				if (a != (c / i) && b != (c / i))ans = min(ans, c / i);
			}
		}
	}
	if (ans == INFL)cout << -1 << endl;
	else cout << ans << endl;


}