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

#define int long long
#define rep(i,l,r) for(int i=(int)(l);i<(int)(r);i++)
#define all(x) (x).begin(),(x).end()
template<class T>bool chmax(T &a,T b){if(a<b){a=b;return 1;}return 0;}
template<class T>bool chmin(T &a,T b){if(a>b){a=b;return 1;}return 0;}

typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<vi> vvi;

const int inf = 1LL<<60;
const int mod = 1e9 + 7;
const double eps = 1e-9;

/*{
  }*/

signed main(){
  int a, b;
  cin >> a >> b;

  int t = a+b;
  set<int> d;
  rep(i, 1, t/i+1){
    if(t%i == 0){
      d.insert(i);
      d.insert(t/i);
    }
  }

  for(int c : d){
    if(c == a or c == b) continue;
    if((a+b)%c == 0 and (b+c)%a == 0 and (c+a)%b == 0){
      cout << c << endl;
      return 0;
    }
  }

  cout << -1 << endl;

  return 0;
}