#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (n); ++i)
using ll = long long;
using ull = unsigned long long;

int main() {
  cin.tie(nullptr)->sync_with_stdio(false);
  ll a, b;
  cin >> a >> b;
  ll x = a + b;
  auto f = [&](ll c) {
    if ((a + b) % c != 0) return false;
    if ((a + c) % b != 0) return false;
    if ((b + c) % a != 0) return false;
    return true;
  };
  ll ans = -1;
  for (ll c = 1; c * c <= x; ++c) {
    if (f(c)) {
      if (ans == -1) ans = c;
      ans = min(ans, c);
    }
    if (x % c == 0 && f(x / c)) {
      if (ans == -1) ans = x / c;
      ans = min(ans, x / c);
    }
  }
  cout << ans << '\n';
  return 0;
}