#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
//#define P pair<ll,ll>
#define FOR(I,A,B) for(int I = int(A); I < int(B); ++I)
#define FORR(I,A,B) for(ll I = ll((B)-1); I >= ll(A); --I)
#define TO(x,t,f) ((x)?(t):(f))
#define SORT(x) (sort(x.begin(),x.end())) // 0 2 2 3 4 5 8 9
#define POSL(x,v) (lower_bound(x.begin(),x.end(),v)-x.begin()) //xi>=v  x is sorted
#define POSU(x,v) (upper_bound(x.begin(),x.end(),v)-x.begin()) //xi>v  x is sorted
#define NUM(x,v) (POSU(x,v)-POSL(x,v))  //x is sorted
#define REV(x) (reverse(x.begin(),x.end())) //reverse
ll gcd(ll a,ll b){if(a%b==0)return b;return gcd(b,a%b);}
ll lcm(ll a,ll b){ll c=gcd(a,b);return ((a/c)*(b/c)*c);}
#define NEXTP(x) next_permutation(x.begin(),x.end())
const ll INF=ll(1e16)+ll(7);
const ll MOD=1000000007LL;
#define out(a) cout<<fixed<<setprecision((a))

vector<ll> enum_div(ll n){//yakusu
	vector<ll> ret;
	for(ll i=1 ; i*i<=n ; ++i){
		if(n%i == 0){
			ret.push_back(i);
			if(i*i!=n)ret.push_back(n/i);
		}
	}
	return ret;
}

int main(){
	ll a,b;
	cin >> a >> b;
	ll ans = INF;
	for(auto c:enum_div(a+b)){
		if( (a+b)%c==0 && (c+b)%a==0 && (a+c)%b==0 ){
			ans = min(ans,c);
		}
	}
	if(ans==INF) ans = -1;
	cout << ans << endl;
}