#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef vector<ll> vl;
typedef long double ld;
typedef vector<ld> vd;
typedef bool bl;
typedef vector<bl> vb;
typedef unordered_map<ll,unordered_map<ll,ll>> graph;

const ll e5 = 1 << 20;
const ll mod = 998244353;
const ll e3 = 1 << 13;
const ll INF = 1ll << 60;

ll a,b;
ll c;
ll ans = INF;

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