#pragma region
#pragma GCC target("avx2")
#pragma GCC optimize("03")
#pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h>
using namespace std; typedef long double ld; typedef long long ll;
typedef unsigned long long ull;
#define endl "\n"
#define FOR(i,a,b) for(int i=(a);i<=(b);i++)
#define rep(i,n) for(int i=0;i<(n);i++)
#define PII pair<int, int>
#define PLL pair<ll, ll>
#define ALL(x) (x).begin(), (x).end()
constexpr int INF=1<<30; constexpr ll LINF=1LL<<60; constexpr ll mod=1e9+7; constexpr int NIL = -1;
template<class T>inline bool chmax(T &a, const T &b) { if (a<b) { a = b; return 1; } return 0; }
template<class T>inline bool chmin(T &a, const T &b) { if (b<a) { a = b; return 1; } return 0; }
#pragma endregion
//-------------------

template<class T>
vector<T> calc_divisor(T n) {
    vector<T> res;
    for (T i = 1; i*i <= n; ++i) {
        if (n % i == 0) {
            res.push_back(i);
            T j = n / i;
            if (j != i) res.push_back(j);
        }
    }
    sort(res.begin(), res.end()); //大きい順
    return res;
}

int main(){
    cin.tie(0); ios::sync_with_stdio(false); //cout << fixed << setprecision(15);
    int a,b; cin >> a >> b;
    auto v = calc_divisor(a+b);
    auto f = [&](int a, int b, int c) {
        bool ans = true;
        if(a==b or b==c or c==a) ans = false;
        if((a+b)%c!=0) ans = false;
        if((b+c)%a!=0) ans = false;
        if((c+a)%b!=0) ans = false;
        return ans;
    };

    for(auto &k: v) {
        if(f(a,b,k)) {
            cout << k << endl;
            return 0;
        }
    }
    cout << -1 << endl;
    return 0;
}