#include<iostream>
#include<vector>
#include<algorithm>
#include<queue>
#include<map>
#include<stack>
#include<cmath>
#include<iomanip>
#include<set>
#include<numeric>
#include<sstream>
#include<random>
#include<cassert>
using namespace std;
typedef long long ll;
#define rep(i, n) for (int i = 0; i < n; ++i)
#define rrep(i, st, n) for (int i = st; i < n; ++i)
using pii = pair<int, int>;
const int inf = 1e9 + 7;
int dy[] = {0, 0, -1, 1, -1, 1, -1, 1};
int dx[] = {1, -1, 0, 0, -1, 1, 1, -1};
#define ceil(a, b) a / b + !!(a % b)

int main() {
    ll a, b; cin >> a >> b;
    for (ll c = 1; c <= 1e9; ++c) {
        if (c > a + b) break;
        if (b > a + c) break;
        if (a > b + c) break;
        if (c == a || c == b) continue;
        if ((a + b) % c != 0) continue;
        if ((b + c) % a != 0) continue;
        if ((c + a) % b != 0) continue;
        cout << c << endl;
        return 0;
    }
    cout << -1 << endl;
    
}