#include <bits/stdc++.h>

#define rep(i, n) for(int i = 0, i##_len = (n); i < i##_len; ++i)
#define repp(i, m, n) for(int i = m, i##_len = (n); i < i##_len; ++i)
#define all(x) (x).begin(), (x).end()
#define clr(ar, val) memset(ar, val, sizeof(ar))
#define sbp(p, n) rep(i, n) { int a, b; scanf("%d %d", &a, &b); p[i] = make_pair(a, b); }
template <class T> bool chmax(T &a, const T &b) { if (a < b) { a = b; return 1; } return 0; }
template <class T> bool chmin(T &a, const T &b) { if (b < a) { a = b; return 1; } return 0; }
int gcd(long a,long b){return b?gcd(b,a%b):a;}
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef pair <int,int> P;
typedef long double ld;

int main(void)
{
  long a, b, k;
  vector<long> yakusu;
  cin >> a >> b;
  for(int i = 1; i * i <= a + b; i++){
    if((a + b) % i == 0){
      yakusu.push_back(i);
    }
  }
  for(long m : yakusu){
    if((a + b) % m == 0 && (a + m) % b == 0 && (b + m) % a == 0){
      cout << m << endl;
      return 0;
    }
  }
  cout << -1 << endl;
  return 0;
}