#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<ll, ll> l_l;

#define EPS (1e-7)
#define INF (1e9)
#define PI (acos(-1))
//const ll mod = 1000000007;

int main() {
    //cout.precision(10);
    cin.tie(0);
    ios::sync_with_stdio(false);
    int A, B;
    cin >> A >> B;
    for(int c = 1; c * c <= (A + B); c++) {
        if(A == c || B == c) 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;
    return 0;
}