#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
#define F first
#define S second
#define pii pair<int, int>
#define eb emplace_back
#define all(v) v.begin(), v.end()
#define rep(i, n) for (int i = 0; i < n; ++i)
#define rep3(i, l, n) for (int i = l; i < n; ++i)
#define chmax(a, b) a = max(a, b)
#define chmin(a, b) a = min(a, b)
#define out(a) cout << a << endl
#define SZ(v) (int)v.size()
#define inf (int)(1e9+7)

// 約数の列挙 O(√n)
vector<int> enum_divisor(int n) {
    vector<int> res;
    for (int i = 1; i * i <= n; i++) {
        if (n % i == 0) {
            res.push_back(i);
            if (i != n / i) res.push_back(n / i);
        }
    }
    return res;
}

int main() {
    int a, b; cin >> a >> b;
    auto v = enum_divisor(a + b);
    int ans = inf;
    rep(i, SZ(v)) {
        if ((v[i] + a) % b != 0 || (v[i] + b) % a != 0) continue;
        if (v[i] == a || v[i] == b) continue;
        chmin(ans, v[i]);
    }
    if (ans == inf) out(-1);
    else out(ans);
}