#include <bits/stdc++.h>

using namespace std;

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  int64_t A, B;
  cin >> A >> B;
  vector<int64_t> C;
  for (int64_t i = 1; i * i < A + B + 1; i++) {
    if ((A + B) % i == 0) {
      C.emplace_back(i);
      C.emplace_back((A + B) / i);
    }
  }
  int64_t res = INT64_C(1) << 50;
  for (const auto &c : C) {
    if (A != c && B != c && (A + B) % c == 0 && (A + c) % B == 0 && (B + c) % A == 0)
      res = min(res, c);
  }
  if (res != INT64_C(1) << 50) {
    cout << res << '\n';
  } else {
    cout << -1 << '\n';
  }
  return 0;
}