#include <iostream>
using namespace std;
int main(int argc, char* argv[])
{
	int n,m;
	cin>>n>>m;
	if (n<=m){
		cout<<1<<endl;
		return 0;
	}
	if (n%2==1){
		cout<<-1<<endl;
		return 0;
	}
	int M=n/2;
	if (M>m){
		cout<<-1<<endl;
		return 0;
	}
	cout<<2<<endl;
	return 0;
}