#include <bits/stdc++.h>
using namespace std;
using lint = long long;
template<class T = int> using V = vector<T>;
template<class T = int> using VV = V< V<T> >;

template<class Z> V<Z> divisor(Z n) {
  V<Z> res;
  Z i;
  for (i = 1; i * i < n; ++i) if (n % i == 0) res.push_back(i), res.push_back(n / i);
  if (i * i == n) res.push_back(i);
  sort(begin(res), end(res));
  return res;
}

int main() {
  cin.tie(nullptr); ios::sync_with_stdio(false);
  int a, b; cin >> a >> b;
  for (int c : divisor(a + b)) {
    if ((a + c) % b == 0 and (b + c) % a == 0) {
      return cout << c << '\n', 0;
    }
  }
  cout << -1 << '\n';
}