#include <iostream>
#include <cmath>
using namespace std;
int main(void){
    unsigned long long a,b,c;
    cin >> a >> b;
    c = a > b ? b : a;
    for(unsigned long long i=2;i<=(a+b)/2;i+=c){
        if( ((a+b)%i==0) && ((a+i)%b==0) && ((b+i)%a==0) && a!=i && b!=i ){
            cout << i;
            return 0;
        }
    }
    cout << -1;
    
}