#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <stack>
#include <queue>
#include <cmath>
#include <tuple>
#include <cstdio>
#include <bitset>
#include <sstream>
#include <iterator>
#include <numeric>
#include <map>
#include <cstring>
#include <set>

using namespace std;

#define DEBUG_ //!!提出時にコメントアウト!!
#ifdef DEBUG_
	#define dump(x)  cerr << #x << " = " << (x) << endl;
#else
	#define dump(x)  ; //何もしない文
#endif

#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)
#define SZ(x) ((int)(x).size()) //unsignedのサイズをint型に変換
#define pb push_back

typedef long long LL;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef pair<LL, LL> PLL;

template <typename T>
std::string printVector(const std::vector<T> &data)
{
    std::stringstream ss;
    std::ostream_iterator<T> out_it(ss, ", ");
    ss << "[";
    std::copy(data.begin(), data.end() - 1, out_it);
    ss << data.back() << "]";
    return ss.str();
}

const int MOD = 1e9+7;
const LL LINF = 1001002003004005006ll;
const int INF = 1001001001;



int main(int argc, char const *argv[])
{
    cin.tie(0);
    ios::sync_with_stdio(false);

    LL A,B; cin >> A >> B;
    vector<LL> cand;

    for(LL i = 1; i*i <= A + B; i++)
    {
        if((A+B)%i == 0)
        {
            cand.pb(i);
            cand.pb((A+B)/i);
        }
    }
    sort(cand.begin(), cand.end());


    for(auto p : cand)
    {
        if((p+B)%A == 0 && (p+A)%B == 0)
        {
            if(p != A && p != B)
            {
                cout << p << endl;
                return 0;

            }
        }


    }
    cout << "-1" << endl;



}