#include <bits/stdc++.h>
#define ft first
#define sc second
#define lb lower_bound
#define ub upper_bound
#define pb push_back
#define pt(sth) cout << sth << "\n"
#define chmax(a, b) {if(a<b) a=b;}
#define chmin(a, b) {if(a>b) a=b;}
#define moC(a, s, b) (a)=((a)s(b)+MOD)%MOD
using namespace std;
typedef long long ll;
typedef pair<ll, ll> P;
static const ll INF=1e18;
static const ll MAX=1e5+7;
static const ll MOD=1e9+7;
ll max(ll a, ll b) {return a>b?a:b;}
ll min(ll a, ll b) {return a<b?a:b;}

map<ll, ll> mp;

void divisor(ll n) {
  for(ll i=1; i*i<=n; i++) {
    if(n%i==0) {
      mp[i]++;
      mp[n/i]++;
    }
  }
}

int main(void) {
  ll a, b;
  cin >> a >> b;
  
  divisor(a+b);
  for(auto it=mp.begin(); it!=mp.end(); it++) {
    ll c=it->ft;
    ll ab=a+b, bc=b+c, ca=c+a;
    
    if(c==a || c==b) continue;
    
    if(ab%c==0 && bc%a==0 && ca%b==0) {
      pt(c);
      return 0;
    }
    
  }
  
  pt(-1);
  
  
}