#include <iostream>
#include <algorithm>
#include <vector>
#include <queue>
#include <iomanip>
#include <cmath>
#include <map>
using namespace std;
using ll = long long;

ll A, B;

int main() {
  cin >> A >> B;
  ll C = A + B;

  vector<int> div;
  for(int i = 1; i * i <= C; ++i) {
    if(C % i == 0) {
      div.push_back(i);
      if(C != i * i) div.push_back(C / i);
    }
  }
  sort(div.begin(), div.end());
  for(int d : div) {
    if(d == A || d == B) continue;
    ll a = A + d;
    ll b = B + d;
    if(a % B == 0 && b % A == 0) {
      cout << d << endl;
      return 0;
    }
  }
  cout << -1 << endl;
  return 0;
}