#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <queue>
#include <string>
#include <map>
#include <set>
#include <stack>
#include <tuple>
#include <deque>
#include <numeric>
#include <bitset>
#include <iomanip>
#include <cassert>
#include <chrono>
#include <random>
#include <limits>
#include <iterator>
#include <functional>
#include <sstream>
#include <complex>
using namespace std;

#define chmax(x, y) x = max(x, y)
#define chmin(x, y) x = min(x, y)

typedef long long ll;
typedef uint64_t ull;
typedef pair<int, int> P;
typedef pair<int, double> Pid;
typedef pair<double, int> Pdi;
typedef pair<ll, int> Pl;
typedef pair<ll, ll> Pll;
typedef pair<int, pair<int, int>> PP;
typedef pair<P, int> PPi;
constexpr double PI = 3.1415926535897932;   // acos(-1)
constexpr double EPS = 1e-9;
constexpr int INF = 1001001001;
constexpr int mod = 1e+9 + 7;
// constexpr int mod = 998244353;

vector<ll> divisor(ll x){
    vector<ll> res;
    for(ll i = 1; i * i <= x; ++i){
        if(x % i == 0){
            res.emplace_back(i);
            if(i * i != x)  res.emplace_back(x / i);
        }
    }
    sort(res.begin(), res.end());
    return res;
}

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

    ll a, b;
    cin >> a >> b;
    vector<ll> div = divisor(a + b);
    int sz = div.size();
    ll ans = -1;
    for(int i = 0; i < sz; ++i){
        ll c = div[i];
        if(c != a && c != b && (b + c) % a == 0 && (c + a) % b == 0){
            cout << c << endl;
            return 0;
        }
    }
    cout << ans << endl;
}