#include<iostream>
#include<algorithm>
#include<string>
#include<vector>
#include<queue>
#include<cmath>
#include<functional>
using namespace std;
int n, m;
int main() {
	cin >> n >> m;
	if (n <= m)cout << "1" << endl;
	else if (n % 2 == 0 && n / 2 <= m)cout << "2" << endl;
	else cout << "-1" << endl;
	return 0;
}