#include <iostream>
using namespace std;

long long int gcd(long long int x,long long int y){
    long long int r;
    if(x == 0 || y == 0)  return 0;

    while((r = x % y) != 0) 
    {
         x = y;
        y = r;
    }
    return y;
}

int main(){
    long long int a,b; cin >> a >> b;
    long long int aa = a / gcd(a,b), bb = b / gcd(a,b);
    if(aa>bb) swap(aa,bb);
    if(aa==1 && bb==2) cout << 3 * gcd(a,b) << endl;
    else if(aa==2 && bb == 3) cout << gcd(a,b) << endl;
    else if(aa==1 && bb == 3) cout << 2 * gcd(a,b) << endl;
    else cout << -1 << endl;
    return 0;
}