#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define fi first
#define se second
int main(){
    ll a,b;
    cin >> a >> b;
    for(ll c=1;c <= a+b;)
    {
        if((a+b)%c == (b+c)%a && (c+a)%b == (a+b)%c && (b+c)%a==0){
            cout << c << endl;
            return 0;
        }

        if((a+c)%b!=0){
            c+=b-(a+c)%b;
        }else if((b+c)%a!=0){
            c+=a-(b+c)%a;
        }else{
            c++;
        }
        
    }
        
    cout << -1 << endl;
}