#include "bits/stdc++.h"
using namespace std;
typedef long long ll;
#define INF (1<<30)
#define INFLL (1ll<<60)
typedef pair<int, int> P;
typedef pair<int, P> E;
#define MOD (1000000007ll)
#define l_ength size

void mul_mod(ll& a, ll b){
	a *= b;
	a %= MOD;
}

void add_mod(ll& a, ll b){
	a += b;
	a %= MOD;
}

int main(void){
	ll a,b,c,s,i;
	cin >> a >> b;
	s = (a+b);
	for(i=1ll; i*i<=s; ++i){
		if(s%i){
			continue;
		}
		c = i;
		if(!(a==b || b==c || c==a || (a+b)%c || (b+c)%a || (c+a)%b )){
			cout << c << endl;
			return 0;
		}
		c = s/i;
		if(!(a==b || b==c || c==a || (a+b)%c || (b+c)%a || (c+a)%b )){
			cout << c << endl;
			return 0;
		}
	}
	cout << -1 << endl;
	return 0;
}