#include<bits/stdc++.h>
using namespace std;
using ll = long long;

#define FOR(i,a,b) for(ll i=(a);i<(b);++i)
#define ALL(v) (v).begin(), (v).end()
#define p(s) cout<<(s)<<endl
#define p2(s, t) cout << (s) << " " << (t) << endl
#define br() p("")
#define pn(s) cout << (#s) << " " << (s) << endl
#define p_yes() p("Yes")
#define p_no() p("No")

const ll mod = 1e9 + 7;
const ll inf = 1e18;

set<ll> devisors(ll N){
    set<ll> se;
    for(int i=1; i*i<=N; i++){
        if(N%i==0){
            se.insert(i);
            se.insert(N/i);
        }
    }
    return se;
}

bool check(ll a, ll b, ll c){
    if(a==b || b==c || c==a) return false;

    if((a+b)%c==0 && (b+c)%a==0 && (c+a)%b==0){
        return true;
    }else{
        return false;
    }
}

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);

    // input
    ll a, b;
    cin >> a >> b;

    auto se = devisors(a+b);
    for(ll c : se){
        if(check(a, b, c)){
            p(c);
            return 0;
        }
    }

    p(-1);
    
    return 0;
}