#include <bits/stdc++.h>
using namespace std;
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) begin(v),end(v)
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; }
using ll = long long;
using pii = pair<int, int>;
constexpr ll INF = 1ll<<30;
constexpr ll longINF = 1ll<<60;
constexpr ll MOD = 1000000007;
constexpr bool debug = false;
//---------------------------------//

std::vector<ll> find_divisor(ll n) {
	assert(n > 0);
	std::vector<ll> divisor;
	ll i = 1;
	for (; i * i <= n; i++) if (n % i == 0) divisor.emplace_back(i);
	int p = divisor.size() - 1;
	if (divisor[p] * divisor[p] == n) --p;
	for (; p >= 0; --p) divisor.emplace_back(n / divisor[p]);
	return divisor;
}

int main() {
	int A, B;
	cin >> A >> B;
	for (ll C : find_divisor(A + B)) {
		if ((A + C) % B != 0) continue;
		if ((B + C) % A != 0) continue;
		if (A == C || B == C) continue;
		cout << C << endl;
		return 0;
	}
	puts("-1");
}