#include <bits/stdc++.h>

using namespace std;

#define rep(i, n) for(int i = 0; i < (int)n; ++i)
#define FOR(i, a, b) for(int i = a; i < (int)b; ++i)
#define rrep(i, n) for(int i = ((int)n - 1); i >= 0; --i)

typedef long long ll;
typedef long double ld;

const int Inf = 1e9;
const double EPS = 1e-9;
const int MOD = 1e9 + 7;

bool check(ll x, ll y, ll z) {
    if ((x + y) % z != 0) return false;
    if ((x + z) % y != 0) return false;
    if ((y + z) % x != 0) return false;

    return true;
}

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(0);
    ll a, b;
    cin >> a >> b;
    ll c = a + b;
    for (ll i = 1; i * i <= c; ++i) {
        if (c % i != 0) continue;
        if (i == a || i == b) continue;
        if (check(a, b, i)) {
            cout << i << endl;
            return 0; 
        }
    }
    cout << -1 << endl;
    
    return 0;
}