#include"bits/stdc++.h"

using namespace std;
using ll = long long;
using ld = long double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;

#define FOR(k,m,n) for(ll (k)=(m);(k)<(n);(k)++)
#define REP(i,n) FOR((i),0,(n))
#define WAITING(str) int str;std::cin>>str;
#define DEBUGING(str) cout<< #str << " " str<<endl

constexpr int INF = (1 << 30);
constexpr ll INFL = (1ll << 60);
constexpr ll MOD = 1000000007;// 10^9+7



int main()
{
	ll A, B, C;
	cin >> A >> B;
	C = A + B;
	ll best = INFL;
	FOR(i, 1, sqrt(C) + 10) {
		if (C%i == 0) {
			if ((A + i) % B != 0)goto next;
			if ((B + i) % A != 0)goto next;
			if (i == A || i == B)goto next;
			best = min(best, i);
		next:;
			ll j = C / i;
			if ((A + j) % B != 0)continue;
			if ((B + j) % A != 0)continue;
			if (j == A || j == B)continue;
			best = min(best, j);
		}
	}
	cout << (best == INFL ? -1 : best) << endl;
	return 0;
}