#include <bits/stdc++.h>
using namespace std;
#define REP(i,a,n) for(int i=(a); i<(int)(n); i++)
#define rep(i,n) REP(i,0,n)
#define FOR(it,c) for(__typeof((c).begin()) it=(c).begin(); it!=(c).end(); ++it)
#define ALLOF(c) (c).begin(), (c).end()
typedef long long ll;
typedef unsigned long long ull;

vector<ll> divisor(ll n){
  vector<ll> ret;
  for(ll i=1; i*i<=n; i++){
    if(n%i==0){
      ret.push_back(i);
      if(i!=n/i) ret.push_back(n/i);
    }
  }
  sort(ret.begin(), ret.end());
  return ret;
}

int main(){
  ll A, B;
  cin >> A >> B;

  vector<ll> div = divisor(A+B);
  ll ret = -1;
  rep(i,div.size()){
    ll x = div[i];
    if(A != x && B != x){
      if((A+B)%x==0 && (A+x)%B==0 && (B+x)%A==0){
        ret = x;
        break;
      }
    }
  }

  cout << ret << endl;
  
  return 0;
}