#include<bits/stdc++.h>
#include <atcoder/all>
#ifdef LOCAL
#include <debug_print.hpp>
#define debug(...) debug_print::multi_print(#__VA_ARGS__, __VA_ARGS__)
#else
#define debug(...) (static_cast<void>(0))
#endif
using namespace atcoder;
using mint=modint1000000007;
using namespace std;
using ll=long long;
using ul=unsigned long long;
int dx[9] = {-1, 1, 0, 0, -1, -1, 1, 1, 0};
int dy[9] = {0, 0, -1, 1, -1, 1, -1, 1, 0};
using Graph=vector<vector<int>>;
ll op(ll a,ll b){return min(a,b);}
ll e(){return 2e9;}
int main(){
	ll A,B;
	cin>>A>>B;
	for(ll i=1;i*i<=A+B;i++){
		if((A+B)%i==0){
			if((A+i)%B==0&&(B+i)%A==0){cout<<i<<endl;return 0;}
			if((A+(A+B)/i)%B==0&&(B+(A+B)/i)%A==0){cout<<(A+B)/i<<endl;return 0;}
		}
	}
	cout<<-1<<endl;
	return 0;
}