#include<iostream>
#include<algorithm>
using namespace std;
using lint=int64_t;

int main()
{
	int n,m;
	cin >> n >> m;

	if(n<=m)cout << 1;
	else if(n%2==0 && n/2<=m)cout << 2;
	else cout << -1;
	cout << endl;
	return 0;
}