#include <bits/stdc++.h>
#define rep(i,n) for(int i=(0);i<(n);i++)

using namespace std;

typedef long long ll;

int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);

	ll a, b;
	cin >> a >> b;

	ll y = a + b;
	vector<ll> ans;
	for(ll x = 1; x * x <= y; x++){
		if(y % x != 0) continue;
		ll c = x;
		if(a != c && b != c && (c + a) % b == 0 && (b + c) % a == 0) ans.push_back(c);
		c = y / x;
		if(a != c && b != c && (c + a) % b == 0 && (b + c) % a == 0) ans.push_back(c);
	}
	
	if(ans.size() == 0){
		cout << -1 << endl;
		return 0;
	}

	sort(ans.begin(), ans.end());
	cout << ans[0] << "\n";
}