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

#define reps(i,a,b) for(int i=(a);i<(b);++i)
#define rep(i,n)   reps(i,0,n)
#define all(x) (x).begin(),(x).end()
#define INF (1000000000)
#define MOD (1000000007)
#define PI (acos(-1))

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

int main(){
  int a,b;
  cin >> a >> b;
  auto v = divisor(a+b);
  sort(all(v));
  int ans=0;
  rep(i,v.size()){
    int c = v[i];
    if((a+c)%b==0&&(b+c)%a==0&&b!=c&&a!=c){
      cout << c << endl;
      return 0;
    }
  }
  cout << -1 << endl;
}