#include <iostream>
//#include<atcoder/all>
#include <numeric>
#include <cmath>
#include <limits>
#include <stdio.h>
#include <iomanip>
#include <string> 
#include <vector>
#include <algorithm>
#include <utility> 
#include <tuple> 
#include <cstdint> 
#include <cstdio> 
#include <map> 
#include <queue>
#include <set> 
#include <stack> 
#include <deque> 
#include <unordered_map> 
#include <unordered_set>
#include <bitset> 
#include <cctype>
//using namespace atcoder;
using namespace std;
using ll = long long;
#define all(A) A.begin(),A.end()
using vll = vector<ll>;
#define rep(i, n) for (long long i = 0; i < (long long)(n); i++)
using Graph = vector<vector<vector<pair<ll, ll>>>>;

ll gcd(ll(a), ll(b)) {
	ll c = a;
	while (a % b != 0) {
		c = a % b;
		a = b;
		b = c;
	}
	return b;
}
ll N, M, Q;
ll an = 0;
vll A, B, C, D;

void next_combination(vll S){
	if (S.size() == N) {
		ll k = 0;
		rep(q, Q) {
			
			if (S[B[q]] - S[A[q]] == C[q])k += D[q];
			
		}
		an = max(an, k);
		//cout << k << endl;
	}
	else {
		vll NE = S;
		if (S.size() == 0) {
			for (ll j = 1; j <= M; j++) {
				NE.push_back(j);
				next_combination(NE);
				NE.pop_back();
			}
		}
		else {
			for (ll j = S[S.size() - 1]; j <= M; j++) {
				
				NE.push_back(j);
				next_combination(NE);
				NE.pop_back();
			}
		}
		
	}
	return;
}


int main(){
	ll A, B;
	cin >> A >> B;
	ll an = 1e18;
	bool K = true;
	for (ll C = 1; C *C <= A + B; C++) {
		if ((A + B) % C == 0) {
			if ((B + C) % A == 0 && (A + C) % B == 0) {
				an = min(an, C);
				K = false;
			}
			ll D = (A + B) / C;
			if ((B + D) % A == 0 && (A + D) % B == 0) {
				an = min(an, D);
				K = false;
			}


		}
	}
	if (!K) {
		cout << an << endl;
	}
	else {
		cout << -1 << endl;
	}
}