#include <iostream>
#include<string>
#include<vector>
#include<algorithm>
using namespace std;
#define REP(i,n) for(int i=0;i<n;i++)
int main(){
cin.tie(0);
ios::sync_with_stdio(false);

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