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

vector<long> div(long n) {
  vector<long> v;
  for (long i = 1; i * i <= n; i++) {
    if (n % i == 0) {
      v.push_back(i);
      if (i * i != n) v.push_back(n / i);
    }
  }
  sort(begin(v), end(v));
  return v;
}

int main() {
  long A, B;
  cin >> A >> B;
  for (auto C : div(A + B)) {
    if (A != C && B != C && (B + C) % A == 0 && (C + A) % B == 0) return cout << C << "\n", 0;
  }
  cout << -1 << "\n";
}