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

int main(){
  int n,m;
  cin>>n>>m;
  if(m>=n){
    cout<<1<<endl;
  }
  else if(n%2==1){
    cout<<-1<<endl;
  }
  else if(n>=m/2){
    cout<<2<<endl;
  }
  else{
    cout<<-1<<endl;
  }
  return 0;
}