/* ---------- STL Libraries ---------- */
// IO library
#include <cstdio>
#include <fstream>
#include <iomanip>
#include <ios>
#include <iostream>

// algorithm library
#include <algorithm>
#include <cmath>
#include <numeric>
#include <random>
#include <cstring>

// container library
#include <array>
#include <bitset>
#include <deque>
#include <map>
#include <unordered_map>
#include <queue>
#include <set>
#include <string>
#include <tuple>
#include <vector>
#include <stack>

/* ---------- Namespace ---------- */
using namespace std;

/* ---------- Type ---------- */
using ll = long long;
#define int ll
#define P pair<ll, ll>

/* ---------- Constants  */
const double PI = 3.141592653589793238462643383279;
const ll MOD = 1e9 + 7;
const int INF = 1LL << 55;

/* v-v-v-v-v-v-v-v-v Main Part v-v-v-v-v-v-v-v-v */
vector<int> divisor(int n) {
    vector<int> ret;

    for (int i = 1; i * i <= n; i++) {
        if (n % i == 0) {
            ret.push_back(i);
            if (i != n / i) ret.push_back(n / i);
        }
    }

    return move(ret);
}

signed main() {
    int A, B;
    cin >> A >> B;
    vector<int> divs = divisor(A + B);
    sort(divs.begin(), divs.end());
    for (int div : divs) {
        if ((B + div) % A == 0 && (div + A) % B == 0) {
            cout << div << endl;
            return 0;
        }
    }
    cout << -1 << endl;
    return 0;
}