#include "bits/stdc++.h"
using namespace std;

#define int long long
#define MAX_N (200005)
#define INF (1LL<<60)
#define debug(x) cerr << #x <<":"<< x << endl;
#define debugVec(x) for(auto p:(x)) cerr << p <<" "; cerr << endl;
#define debugVecPair(x) for(auto p:(x)) cerr << #x <<": "<< p.first <<" "<< p.second << endl;
#define debugIdx(i, x) cerr << #i <<" : "<< i <<" "<< #x <<" : "<< x << endl;
const int MOD = (int)1e9+7;

// 約数の列挙O(√n)
vector<int> divisor(int n){
    vector<int> res;
    for(int i=1; i*i<=n; i++){
        if(n%i==0){
            res.push_back(i);
            // n の約数が i の時、n/i も n の約数になる
            if(i!=n/i) res.push_back(n/i);
        }
    }
    return res;
}

signed main() {
    int A, B;
    cin >> A >> B;
    vector<int> C;
    C = divisor(A+B);
    int ans=INF;
    for(int i=0; i<C.size(); i++){
        if((A+C[i])%B==0 && (B+C[i])%A==0){
            ans = min(ans, C[i]);
        }
    }
    cout << (ans==INF?-1:ans) << endl;
}