#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i = 0; i < (n);i++)
#define sz(x) int(x.size())
typedef long long ll;
typedef pair<int,int> P;

int main(){
  ll a, b;
  cin >> a >> b;
  for (ll i = 1; i * i <= a + b; i++) {
    if (a == i || b == i) continue;
    if ( (a + b) % i != 0) continue;

    if ((i + b) % a == 0 && (i + a) % b == 0) {
      cout << i << endl;
      return 0;
    }
    ll j = (a + b) / i;

    if ((j + b) % a == 0 && (j + a) % b == 0) {
      cout << j << endl;
      return 0;
    }  
  }
  cout << -1 << endl;
  return 0;
}