#include <iostream>
int main(){
	int n,m;
	std::cin>>n>>m;
	if(n<=m){
		std::cout<<1<<std::endl;
		return 0;
	}
	if(n<=m*2){
		if(n%2==0)std::cout<<2<<std::endl; else std::cout<<-1<<std::endl;
		return 0;
	}
	std::cout<<-1<<std::endl;
	return 0;
}