#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using vi = vector<int>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
using vll = vector<ll>;
using vvll = vector<vll>;
using vvvll = vector<vvll>;
using vs = vector<string>;
using pll = pair<ll, ll>;
using vp = vector<pll>;
#define rep(i, n) for(ll i = 0; i < (n); i++)
#define repr(i, a, b) for(ll i = (a); i < (b); i++)
#define ALL(a) (a).begin(), (a).end()
#define SZ(x) ((ll)(x).size())
const ll MOD = 1000000007;
const ll INF = 100000000000000000LL;
inline ll GCD(ll a, ll b){ return b?GCD(b, a % b):a; }
inline ll LCM(ll a, ll b){ return a/GCD(a, b)*b; }
inline ll powint(ll x, ll y){ ll r=1; while(y){ if(y&1) r*=x; x*=x; y>>=1; } return r; }
inline ll powmod(ll x, ll y, ll m = MOD){ ll r=1; while(y){ if(y&1) r*=x; x*=x; r%=m; x%=m; y>>=1; } return r; }
template<class T>bool chmax(T &a, const T &b){ if(a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b){ if(b<a) { a=b; return 1; } return 0; }
#ifdef OJ_LOCAL
#include "dump.hpp"
#else
#define dump(...) ((void)0)
#endif

/* 条件
 * A != B (制約)
 * B != C
 * C != A
 * (A+B) % C == 0 CはA+Bの約数
 * (B+C) % A == 0
 * (C+A) % B == 0
 * Cは、これらの条件を満たすものの中で最小
 * 
 */
vll divisor(ll x){
    vll ret;
    for(ll i = 1; i*i <= x; ++i){
        if(x % i == 0){
            ret.emplace_back(i);
            if(i*i != x){
                ret.emplace_back(x/i);
            }
        }
    }
    sort(ALL(ret));
    return ret;
}

int main(){
    cin.tie(0); ios::sync_with_stdio(false);
    cout << fixed << setprecision(15);
    ll a, b;
    cin >> a >> b;
    vll div = divisor(a+b);
    for(auto&& c: div){
        if((b+c)%a == 0 && (c+a)%b == 0 && b != c && c != a){
            cout << c << endl;
            return 0;
        }
    }
    cout << -1 << endl;
    return 0;
}