//
//
//  Codeforces: ganariya
//  AtCoder: ganariya2525
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//include
//------------------------------------------
#include <vector>
#include <list>
#include <map>
#include <unordered_map>
#include <climits>
#include <set>
#include <unordered_set>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>
#include <queue>
#include <random>
#include <complex>
#include <regex>
#include <locale>
#include <random>
#include <type_traits>

using namespace std;

#define SHOW_VECTOR(v) {std::cerr << #v << "\t:";for(const auto& xxx : v){std::cerr << xxx << " ";}std::cerr << "\n";}
#define SHOW_MAP(v){std::cerr << #v << endl; for(const auto& xxx: v){std::cerr << xxx.first << " " << xxx.second << "\n";}}

using LL = long long;

//~~~~~~~~~~~~~~~~~~~~~_(＾～＾ ｣ ∠)_~~~~~~~~~~~~~~~~~~~~~

vector<LL> get_divisors(LL x) {
    vector<LL> ret;
    for (LL i = 1; i * i <= x; i++) {
        if (x % i == 0) {
            ret.push_back(i);
            if (x / i != i) {
                ret.push_back(x / i);
            }
        }
    }
    sort(ret.begin(), ret.end());
    return ret;
}

int main() {

    LL A, B;
    cin >> A >> B;

    LL D = A + B;
    auto divisors = get_divisors(D);

    LL ans = LONG_LONG_MAX;

    for (auto C: divisors) {
        if ((B + C) % A == 0 && (A + C) % B == 0 && A != B && B != C && A != C) {
            ans = min(ans, C);
        }
    }

    if (ans == LONG_LONG_MAX) cout << -1;
    else cout << ans;

    return 0;
}