#include <bits/stdc++.h>

#define rep(i, n) for(int i = 0, i##_len = (n); i < i##_len; ++i)
long gcd(long a,long b){return b?gcd(b,a%b):a;}
using namespace std;

int main(void)
{
  long a, b, k;
  vector<long> yakusu;
  cin >> a >> b;
  for(int i = 1; i * i <= a + b; i++){
    if((a + b) % i == 0){
      yakusu.push_back(i);
    }
  }
  int len = yakusu.size();
  rep(i, len){
    yakusu.push_back((a + b) / yakusu[i]);
  }
  for(long m : yakusu){
    if(m != a && m != b
      && (a + b) % m == 0 && (a + m) % b == 0 && (b + m) % a == 0){
      cout << m << endl;
      return 0;
    }
  }
  cout << -1 << endl;
  return 0;
}