#include <bits/stdc++.h>

using namespace std;

#define REP(i,n) for(int i=0; i<(int)(n); i++)
#define FOR(i,b,e) for (int i=(int)(b); i<(int)(e); i++)
#define ALL(x) (x).begin(), (x).end()

const double PI = acos(-1);

vector<long long> divisor(long long n) {
  vector<long long> ret;
  for (long long i = 1; i * i <= n; i++) {
    if (n % i == 0) {
      ret.push_back(i);
      if (i != n / i)
        ret.push_back(n / i);
    }
  }
  return ret;
}

int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);

  int a, b;
  cin >> a >> b;

  int ret = -1;
  for (int c: divisor(a + b)) {
    if (a != c && b != c && (a + c) % b == 0 && (b + c) % a == 0) {
      ret = c;
      break;
    }
  }

  cout << ret << endl;
  
  return 0;
}