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

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

  vector<long> ds;
  long sum = a + b;
  for (int i=1; i<=sqrt(sum); i++) {
    if (sum % i == 0) {
      ds.push_back(i);
      if (i != sum / i) {
        ds.push_back(sum / i);
      }
    }
  }
  sort(ds.begin(), ds.end());
  for (long c: ds) {
    if (c == a || c == b) {
      continue;
    }
    if ((a + b) % c != 0) {
      continue;
    }
    if ((c + a) % b != 0) {
      continue;
    }
    if ((b + c) % a != 0) {
      continue;
    }
    cout<<c<<endl;
    return 0;
  }
  cout<<-1<<endl;
}